Medial Unicondylar Knee Replacement

1. High-Yield Summary

Medial unicondylar knee replacement (UKA), also termed unicompartmental knee arthroplasty, is an established, bone-conserving, ligament-preserving surgical reconstruction for isolated medial compartment osteoarthritis or spontaneous osteonecrosis of the knee (SONK). Unlike total knee arthroplasty (TKA), which alters normal knee kinematics through cruciate ligament sacrifice and non-physiologic joint line realignment, medial UKA resurfaces only the damaged medial articular cartilage and subchondral bone. This restores near-normal native knee kinematics, preserves proprioceptive feedback from the intact cruciate and collateral linkages, accelerates functional rehabilitation, and provides superior patient-reported satisfaction when performed for appropriately selected indications.

• Primary Clinical Indication: End-stage isolated medial compartment osteoarthritis (anteromedial osteoarthritis, AMOA) characterized by full-thickness bone-on-bone contact, a functionally intact and competent anterior cruciate ligament (ACL), fully preserved full-thickness articular cartilage throughout the lateral compartment, and a passively correctable intra-articular varus deformity.
• The Single Critical Move: Restoring the native medial joint line height and matching the rectangular flexion and extension gaps while leaving the knee in slight constitutional undercorrection (typically 2° to 4° of native varus). The superficial and deep medial collateral ligaments must never be surgically released or elevated from bone, and valgus overcorrection must be strictly avoided to prevent devastating mechanical overload of the unreplaced lateral compartment.
• Neurovascular Danger Zones and Retraction Safety: The infrapatellar branch of the saphenous nerve (IPBSN) traverses horizontally across the proximal anteromedial tibia and must be protected during superficial incision and subcutaneous dissection to prevent painful neuroma formation. The popliteal neurovascular bundle lies directly posterior to the tibial plateau separated by the posterior capsule and popliteus tendon; retractor placement must remain strictly subperiosteal and intra-capsular, and oscillating saw blades must never penetrate past the posterior tibial cortex.
• Catastrophic Technical Errors to Avoid: First, overcutting or notching the junction between the vertical sagittal cut and horizontal tibial resection, which creates a sharp stress riser and causes periprosthetic medial tibial plateau fracture. Second, placing the vertical sagittal cut too lateral and transecting the medial tibial spine or the tibial insertion footprint of the anterior cruciate ligament. Third, overstuffing the medial compartment with an oversized bearing or inadequate bone resection, driving the mechanical axis into valgus and precipitating rapid lateral compartment osteoarthritis. Fourth, failing to debride extruded polymethylmethacrylate (PMMA) bone cement from the posterior condylar recess, predisposing to third-body abrasive wear, posterior impingement, and mobile bearing dislocation.

2. Indications and Patient Selection

Successful long-term outcomes in medial unicompartmental knee arthroplasty depend on disciplined patient selection. The clinical diagnosis must confirm isolated medial joint pathology where pain, radiographic disease, and physical examination findings correspond precisely to the medial tibiofemoral compartment.

Classic Kozinn and Scott Selection Criteria

In 1989, Kozinn and Scott established landmark selection criteria designed to optimize implant survivorship in fixed-bearing unicompartmental arthroplasty. These historical criteria defined a restrictive patient cohort:

• Age greater than 60 years.
• Body weight less than 82 kg (180 lbs).
• Low physical activity demands (sedentary or light recreational lifestyles).
• Absence of resting joint pain, inflammatory symptoms, or generalized pain syndromes.
• Flexion contracture less than 5° with preoperative active knee flexion of at least 90°.
• Coronal angular deformity less than 10° varus that is passively correctable to neutral under manual valgus stress.
• Intact anterior cruciate ligament and functionally competent medial and lateral collateral ligaments.
• Normal articular cartilage in the lateral tibiofemoral and patellofemoral compartments.

Modern Expanded Indications and Consensus Guidelines

Over the past three decades, advancements in mobile-bearing and fixed-bearing kinematics, robotic navigation, and surgical technique have substantially refined and expanded these initial boundaries. Contemporary consensus guidelines support the following evolutions:

• Chronological Age: Chronological age below 60 years is no longer an absolute contraindication. Active younger patients treated with UKA demonstrate superior kinematics, faster return to work, and higher functional scores than age-matched TKA cohorts. Although younger patients have higher cumulative revision rates over 15 to 20 years, revision from a primary UKA to a standard primary TKA is straightforward and achieves excellent clinical outcomes.
• Body Mass Index (BMI) and Weight: Extensive clinical series from the Oxford group and international registries have demonstrated that a BMI greater than 30 or 35 kg/m², or body weight exceeding 82 kg, does not independently compromise implant survivorship when components are accurately positioned and mechanical undercorrection is maintained.
• Patellofemoral Joint Degeneration: Asymptomatic patellofemoral cartilage wear, trochlear groove chondromalacia, and medial facet osteophytes are not contraindications to medial UKA. Only severe, bone-on-bone lateral patellofemoral facet eburnation, lateral facet bone loss, or symptomatic patellar maltracking constitute contraindications.
• Chondrocalcinosis: Incidental radiographic chondrocalcinosis without clinical attacks of acute pseudogout or diffuse panarticular cartilage degradation does not degrade UKA survivorship.
• Inviolable Core Mandates: Despite expanding indications, three core biomechanical criteria remain strictly non-negotiable: (1) full-thickness articular cartilage loss isolated to the medial compartment with intact lateral cartilage, (2) an intact, functionally competent anterior cruciate ligament, and (3) a passively correctable coronal varus deformity demonstrating medial collateral ligament competence without fixed soft-tissue contracture.

3. Contraindications

Contraindications must be identified during preoperative workup and re-verified by direct visual inspection upon opening the joint. Proceeding with UKA in the presence of these factors predisposes to early mechanical loosening, rapid lateral compartment breakdown, or catastrophic instability.

Absolute Contraindications

• Active Infection: Active local septic arthritis, chronic periarticular osteomyelitis, or uncontrolled systemic bacteremia.
• Inflammatory Arthropathy: Rheumatoid arthritis, psoriatic arthritis, ankylosing spondylitis, or systemic lupus erythematosus involving the knee. Chronic synovial pannus inevitably invades the unreplaced compartments, resulting in panarticular chondrolysis and premature failure.
• Anterior Cruciate Ligament (ACL) Incompetence: Absent, ruptured, or functionally attenuated ACL. Without an intact ACL, the normal femoral rollback is lost, allowing the medial femoral condyle to translate abnormally posterior on the tibia. This concentrates contact forces on the posterior tibial rim, resulting in eccentric loading, early tibial loosening, severe pain, and mobile bearing dislocation.
• Lateral Compartment Cartilage Loss: Full-thickness articular cartilage loss, subchondral bone exposure, or marginal osteophytosis in the lateral compartment.
• Fixed, Non-Correctable Coronal Deformity: Rigid varus deformity exceeding 10° to 15° that cannot be passively corrected to neutral on physical examination or stress radiography, signifying fixed contraction of the medial collateral ligament or posteromedial capsule.
• Severe Motion Restriction: Fixed flexion contracture greater than 10° to 15°, or maximum knee flexion less than 90°.
• Tricompartmental Osteoarthritis: Generalized or diffuse multicompartmental osteoarthritic involvement.

Relative Contraindications

• Symptomatic Lateral Patellofemoral Disease: Severe anterior knee pain provoked by stair climbing or prolonged sitting, accompanied by bone-on-bone lateral patellar facet contact, subchondral sclerosis, or patellar subluxation.
• Previous High Tibial Osteotomy (HTO): Altered proximal tibial metaphyseal geometry, hardware retention, or uncorrected multiplanar deformities that distort anatomical reference axes.
• Neuromuscular and Neuropathic Disorders: Neuropathic arthropathy (Charcot joint), severe parkinsonian tremors, active muscular dystrophies, or severe lower extremity ataxia compromising joint stabilization.
• Excessive Coronal Varus Deformity: Constitutional varus exceeding 15°, which challenges conservative undercorrection goals and elevates shear loads on the fixation interface.
• Heavy Repetitive Impact Labor: Occupations involving extreme dynamic impact loads or heavy manual lifting that significantly exceed physiologic joint contact pressures.

4. Pertinent Surgical Anatomy & Biomechanics

A comprehensive understanding of periarticular ligamentous anatomy, contact biomechanics, and native joint kinematics is mandatory. Medial UKA reconstructs an intrinsically constrained biological system; success hinges on preserving native soft-tissue isometry rather than altering it.

Medial Collateral Ligament Complex: Superficial and Deep Anatomy

The medial collateral ligament complex comprises two distinct structural layers:

• Deep Medial Collateral Ligament (dMCL): Consists of meniscofemoral and meniscotibial (coronary) ligaments, which attach directly to the mid-body of the medial meniscus and insert along the medial tibial plateau margin just 1 to 2 mm distal to the articular cartilage rim. The dMCL acts as a secondary restraint to valgus stress and external tibial rotation.
• Superficial Medial Collateral Ligament (sMCL): Originates from the medial femoral epicondyle, passes superficial to the deep capsule, and inserts broadly onto the anteromedial aspect of the tibia approximately 4.5 to 5.0 cm distal to the joint line, lying deep to the pes anserinus tendons. The sMCL is the primary stabilizer against valgus displacement throughout the arc of motion.
• The Inviolable Surgical Rule: Unlike total knee arthroplasty, which routinely performs extensive subperiosteal ligament releases to correct alignment, medial UKA forbids any surgical release or peeling of the superficial or deep MCL from the tibia or femur. The joint space is decompressed purely by removing osteophytes and resecting damaged bone. Releasing the MCL creates irreparable iatrogenic coronal laxity, disrupts kinematic stability, causes asymmetric edge loading, and induces mobile bearing dislocation.

Anterior Cruciate Ligament Integrity and Anteromedial OA (AMOA) Pathoanatomy

The anterior cruciate ligament originates from the posteromedial aspect of the lateral femoral condyle and inserts onto the anterior intercondylar area of the tibia, immediately anterior and lateral to the medial tibial spine. In anteromedial osteoarthritis (AMOA), cartilage erosion is confined to the anterior two-thirds of the medial tibial plateau and the corresponding anteromedial femoral condyle during early-to-mid flexion. The posterior third of the medial tibial plateau retains healthy, full-thickness hyaline cartilage because an intact, functional ACL guides physiological femoral rollback and prevents excessive posterior translation of the femur. If the ACL is ruptured, paradoxical anterior tibial translation shifts femoral contact onto the posterior tibial margin, destroying posterior cartilage and creating severe shear stress that leads to early implant loosening.

Posterior Tibial Slope and Sagittal Kinematics

The native medial tibial plateau has a natural posterior slope averaging 5° to 7° (range 3° to 10°). In fixed-bearing medial UKA, resecting the tibia to match the patient’s individual native posterior slope restores physiological flexion gap tension and extensor mechanism efficiency. In mobile-bearing designs (such as the Oxford knee), reproducing a 5° to 7° posterior slope is essential. If the posterior slope is cut too steep (greater than 8° to 10°), shear forces on the anterior cruciate ligament increase dramatically, the anterior joint space opens, and the risk of anterior bearing spinout or dislocation escalates. Conversely, an under-sloped resection (less than 3°) closes down the flexion gap relative to the extension gap, restricts deep knee flexion, and risks posterior bearing impingement.

Joint Line Maintenance and Contact Stresses

Restoring the medial joint line to within 1 to 2 mm of its premorbid level is essential for preserving collateral ligament isometry. Over-resecting the tibia drops the joint line, necessitating a thick polyethylene insert that alters collateral tension vectors and weakens proximal tibial cancellous bone support. Conversely, under-resecting bone or inserting an excessively thick implant elevates the joint line, tensions the medial soft tissues excessively, impairs complete extension, and shifts load to the lateral compartment.

5. Preoperative Planning & Radiographic Evaluation

Precise preoperative radiographic templating confirms patient candidacy, establishes bone resection planes, and prevents intraoperative sizing errors.

Essential Radiographic Series

• Weight-Bearing Anteroposterior (AP) View: Obtained with the patient standing and knees in full extension. Demonstrates medial compartment joint space narrowing, subchondral sclerosis, and confirms preservation of normal joint space height in the lateral compartment.
• Rosenberg View (45° Flexion Posteroanterior Weight-Bearing): The essential view for detecting anteromedial osteoarthritis. Because the knee is flexed 45°, contact shifts to the posterior femoral condyle and posteromedial tibial plateau, revealing full-thickness cartilage wear that is frequently obscured on extension AP radiographs.
• True Lateral Radiograph: Evaluates native posterior tibial slope, patellofemoral joint space, and the presence of healthy, preserved articular cartilage on the posterior third of the medial tibial plateau. Absence of posterior cartilage loss serves as an indirect radiographic confirmation of ACL functional integrity.
• Axial Patellar View (Merchant or Skyline): Evaluates patellofemoral joint congruence, lateral facet tilt, and distinguishes harmless medial facet osteophytes from true bone-on-bone lateral facet eburnation.
• Full-Length Standing Bilateral Scanogram (Hip-to-Ankle): Quantifies the overall mechanical axis (hip-knee-ankle angle), measures anatomical femoral and tibial valgus/varus angles, and rules out significant extra-articular diaphyseal deformities that would complicate unicondylar reconstruction.

Stress Radiography and Cartilage Assessment

Stress radiography is the definitive diagnostic modality for verifying unicompartmental eligibility:

• Valgus Stress Radiograph at 20° to 30° Flexion: Performed manually or using a calibrated stress device. This view opens the medial compartment, demonstrating that the varus deformity is intra-articular and passively correctable to neutral. Crucially, the applied valgus load compresses the lateral compartment; visualization of a full-thickness, normal-height lateral joint space confirms intact lateral articular cartilage. Failure of the medial compartment to open or persistent lateral joint space narrowing under valgus stress indicates fixed contracture or lateral arthrosis, mandating conversion to total knee arthroplasty.
• Varus Stress Radiograph: Confirms full-thickness medial cartilage erosion and verifies collateral ligament competence.
• Role of Advanced Imaging (MRI / CT): Magnetic resonance imaging (MRI) is rarely necessary in patients with classic clinical AMOA and diagnostic stress radiographs. MRI is selectively reserved for cases with ambiguous ACL integrity, suspected meniscal root avulsions without radiographic arthrosis, or to differentiate spontaneous osteonecrosis of the knee (SONK) from subchondral insufficiency fractures. Preoperative CT scans are routinely acquired when employing robotic-assisted or patient-specific surgical navigation platforms.

Templating and Sizing Considerations

Digital templating determines the optimal component dimensions:

• Tibial Baseplate Sizing: The tibial component must maximize cortical rim contact along the medial, posterior, and anteromedial cortices of the medial plateau without overhang. Medial overhang causes painful friction against the deep MCL, whereas anterior or anteromedial overhang irritates the pes anserinus tendons and infrapatellar fat pad. Lateral sizing must provide adequate clearance from the medial tibial spine.
• Femoral Component Sizing: Sizing matches condylar width and radius of curvature. The femoral component must not overhang anteriorly into the patellofemoral articulation, which causes painful patellar facet impingement in deep flexion, nor overhang medially where it can impinge on the soft tissues.

6. Patient Positioning & Room Setup

Operating room configuration and patient positioning must allow fluid, unrestricted intraoperative manipulation of the operative limb from full extension to hyperflexion.

Patient Positioning and Leg Handling Approaches

• Hanging Leg Technique: The patient is positioned supine on a standard operating table with the break positioned at mid-thigh. A lateral thigh support post or padded thigh clamp is secured at mid-thigh, and the foot of the table is lowered 90°. The operative limb hangs freely under gravity at 90° of flexion. Gravity provides natural distraction of the medial tibiofemoral joint, improving visualization without requiring excessive manual retraction. The surgeon controls the leg by supporting the foot between their knees, allowing seamless, rapid transitions from 0° extension to 110° flexion during trialing and gap assessment.
• Leg Holder Technique: Alternatively, a commercial adjustable leg holder (e.g., Alvarado or circumferential thigh holder) is mounted on a flat radiolucent table. The leg must be cycled through a complete arc of motion from 0° full extension to 120° deep flexion prior to sterile draping, verifying that the holder does not collide with the heel, calf, or pelvis.
• Tourniquet Placement: A contoured, well-padded pneumatic tourniquet is placed high on the proximal thigh. After limb exsanguination with an Esmarch bandage, the tourniquet is inflated to 200 to 250 mmHg (or 100 mmHg above systolic pressure). Alternatively, a tourniquetless technique utilizing epinephrine-containing local infiltration analgesia (LIA) may be employed to facilitate immediate postoperative quadriceps recovery.

Equipment and Visualization Corridor

• Specialty Instrumentation: Dedicated unicompartmental micro-retractor sets, including narrow curved Hohmann retractors, Z-retractors, Langenbeck retractors, and a specialized blunt collateral ligament protector designed to seat deep to the deep MCL.
• Powered Cutting Tools: High-frequency oscillating saw with narrow (typically 12 to 15 mm) saw blades for the horizontal tibial resection, and a reciprocating saw with fine sagittal blades for the vertical tibial cut to prevent overcutting. High-speed spherical burrs are prepared for robotic or condylar sculpturing systems.
• Lighting Corridor: Dual overhead surgical lights are positioned to project a concentrated light beam directly down the medial joint axis, illuminating the posterior condylar recess without shadow interference.

7. Surgical Approach & Exposure

Medial UKA utilizes a minimally invasive, soft-tissue-preserving approach that exposes the medial compartment while minimizing extensor mechanism disruption.

Minimally Invasive Medial Parapatellar Arthrotomy

• Skin Incision: With the knee flexed to 90°, a longitudinal incision 6 to 8 cm in length is made along the medial border of the patella. The incision extends from the superior pole of the patella to approximately 2 cm distal to the medial tibial joint line, positioned halfway between the medial margin of the patella and the medial femoral epicondyle.
• Capsular Exposure: Full-thickness skin flaps are developed without undermining the subcutaneous fat. The medial retinaculum and joint capsule are incised longitudinally, leaving a robust 3 to 4 mm cuff of capsulofascial tissue on the medial border of the patella to facilitate watertight closure. Distally, the arthrotomy extends along the medial border of the patellar tendon, terminating just medial to the tibial tubercle.

Alternative Subvastus and Midvastus Exposures

• Subvastus Approach: The inferior muscular border of the vastus medialis obliquus (VMO) is mobilized and lifted bluntly off the medial intermuscular septum. The arthrotomy enters beneath the VMO without dividing any muscle fibers. This approach preserves extensor mechanism vascularity and neuromuscular integrity, virtually eliminating postoperative quadriceps extensor lag.
• Midvastus Approach: Employs an oblique split through the VMO muscle fibers for 1.5 to 2.0 cm in line with their natural trajectory. This provides enhanced exposure in muscular or obese patients while sparing the rectus femoris tendon and patellar vascular plexus.

Patellar Subluxation Without Eversion

The patella is gently subluxated laterally into the lateral trochlear groove using a Z-retractor or Langenbeck retractor; it is strictly never everted. Patellar eversion places extreme traction on the patellar tendon insertion, risks avulsion of the tibial tubercle, strains the quadriceps tendon, and compromises postoperative patellofemoral tracking. A small portion of the hypertrophic anterior infrapatellar fat pad (Hoffa’s fat pad) is excised to improve visualization of the anterior tibial plateau and intercondylar notch, while preserving the main vascular supply to the patellar tendon.

Neurovascular Danger Zones and Safe Retraction Corridors

• Infrapatellar Branch of the Saphenous Nerve (IPBSN): Traverses through the sartorial fascia and crosses the proximal medial tibia from superior-medial to inferior-lateral. Incisions extending too far distal or aggressive medial subcutaneous dissection risk lacerating this nerve, resulting in painful neuroma formation and anterior knee numbness. Retractors placed over the medial tibial metaphysis must be kept subperiosteal.
• Safe Medial Retraction Corridor: A blunt, curved collateral ligament retractor is placed subperiosteally deep to the deep medial collateral ligament, seated directly against the medial tibial bone cortex. This shields the deep and superficial MCL fibers from inadvertent laceration by oscillating saw blades or motorized burrs.
• Posterior Neurovascular Danger Zone: The popliteal artery, popliteal vein, and tibial nerve lie directly posterior to the central-lateral aspect of the posterior capsule. A narrow blunt Hohmann retractor is positioned strictly within the joint capsule over the posterior rim of the medial tibial plateau to depress the posterior capsule away from the saw blade. Horizontal saw cuts must never penetrate beyond the posterior tibial cortical rim.
• Mandatory Intraoperative Decision Gate: Prior to performing any irreversible bone resections, the surgeon must perform direct intraoperative inspection of three critical structures: (1) The anterior cruciate ligament: must be completely intact, taut under hook inspection, and demonstrate normal synovial coverage; (2) The lateral tibiofemoral compartment: must exhibit intact, full-thickness articular cartilage with normal meniscus architecture; (3) The patellofemoral joint: must lack full-thickness lateral facet bone loss or eburnation. If any of these structures demonstrate end-stage disease or functional incompetence, unicompartmental arthroplasty must be immediately aborted and converted to total knee arthroplasty.

8. Technique & Bone Preparation

Bone preparation proceeds in an orderly, reproducible sequence. Resection planes must be calibrated to match the prosthetic geometry while conserving native subchondral bone stock.

Step 1: Exposure, Soft-Tissue Mobilization, and Osteophyte Excision

Incise and excise the anterior horn and anterior third of the medial meniscus. Expose the anterior and medial margins of the medial tibial plateau, taking care not to detach the deep MCL insertion. Debride all peripheral medial tibial, medial femoral, and intercondylar notch osteophytes using narrow rongeurs, osteotomes, or a motorized burr. Intercondylar osteophytes cause mechanical impingement against the ACL, whereas peripheral medial marginal osteophytes tent the medial collateral ligament, creating false soft-tissue tension. Meticulous osteophyte debridement restores the true anatomical length of the MCL and reveals true joint laxity before bone cuts are established.

Step 2: Tibial Resection and Posterior Slope Alignment

Mount the extramedullary tibial alignment guide along the anterior crest of the tibia. Align the coronal plane parallel to the tibial mechanical axis (0° varus/valgus). Set the sagittal posterior slope on the guide to replicate the patient’s native medial tibial slope (typically 5° to 7°). Position the resection stylus to reference the deepest point of the medial tibial chondral defect, setting the horizontal resection 2 to 4 mm below the eburnated bone bed (or approximately 6 to 8 mm below the preserved posterior cartilage rim). This ensures adequate clearance for the minimum composite baseplate and polyethylene thickness (typically 8 to 9 mm) without sacrificing strong subchondral cancellous bone. Secure the cutting block with headless fixation pins. Osteotomize the horizontal tibial surface using an oscillating saw, holding the blade perfectly flat against the cutting block. Verify that the saw blade does not plunge through the posterior cortex or encroach upon the tibial spine.

Step 3: Sagittal Tibial Cut and Sagittal Margin Preservation

Align the vertical sagittal cutting guide parallel to the anteroposterior (AP) mechanical axis of the tibia, pointing toward the intercondylar notch and center of the femoral head. Locate the sagittal cut precisely 1 to 2 mm medial to the apex of the medial tibial spine. This landmark ensures that the entire tibial insertion footprint of the anterior cruciate ligament is completely preserved. Execute the vertical cut using a fine reciprocating saw blade, directing the saw downward until it intersects the horizontal resection floor. The reciprocating saw blade must stop exactly at the horizontal plane; never propagate the cut past the horizontal resection floor into the anterior cortex. Mobilize and extract the resected medial tibial bone wafer with a narrow osteotome. Excise the posterior horn of the medial meniscus under direct vision, leaving the posterior capsule clean and intact.

Step 4: Femoral Distal and Posterior Condylar Preparation

Cannulate the distal medial femoral condyle with an intramedullary alignment rod, or position the femoral guide referencing the distal condylar articular surface in robotic or extramedullary systems. Resect the distal femoral condyle with the knee in 0° full extension, removing a bone slice corresponding precisely to the distal thickness of the femoral implant (typically 4 to 6 mm). Flex the knee to 90° and mount the posterior femoral resection and chamfer guide. Resect the posterior medial femoral condyle, matching the posterior component thickness to ensure flexion gap balance. Drill the condylar fixation lug holes and chamfers. Inspect the anterior transition: the anterior border of the femoral component must sit perfectly flush with the anterior condylar articular cartilage, with no anterior overhang into the trochlear groove.

Critical Move

The irreversible procedural crux of medial UKA is the precise execution of the vertical sagittal tibial cut and its exact termination at the horizontal resection plane. If the sagittal cut extends deeper than the horizontal cut, an acute stress-concentration notch is created in the proximal tibial metaphysis, which markedly reduces load-bearing capacity and precipitates an early periprosthetic medial tibial plateau fracture upon postoperative weight-bearing. Furthermore, angling the sagittal cut too laterally violates the anterior cruciate ligament footprint, causing iatrogenic instability and rapid implant failure. Tactile confirmation that the bone wafer releases cleanly without downward sawing into the cortex is the mandatory checkpoint for technical success.

9. Component Trialing, Gap Balancing & Cementation

Gap balancing in UKA restores stability across the entire arc of motion by adjusting bone resections to match native ligament tension.

Flexion and Extension Gap Symmetry

Trial tibial baseplates, trial femoral components, and trial bearings are inserted. The knee is evaluated at 0° full extension and at 90° of flexion using feeler gauges or calibrated tension blocks:

• Equal, Rectangular Gaps: The extension gap (0°) and flexion gap (90°) must be rectangular and mutually balanced.
• Tight in Flexion, Balanced in Extension: Increase the posterior tibial slope by recutting the tibial plateau, or resect additional posterior femoral condyle bone.
• Tight in Extension, Balanced in Flexion: Resect an additional 1 to 2 mm of distal femur. Never recut the tibia in this scenario, as that would inappropriately enlarge the flexion gap.
• Tight in Both Flexion and Extension: Resect an additional 1 to 2 mm from the horizontal tibial cut, preserving the set posterior slope.
• Loose in Both Flexion and Extension: Upsize the polyethylene bearing to a thicker insert.

Avoidance of Overcorrection and Lateral Overload

The cardinal rule of medial unicompartmental knee replacement is the strict avoidance of valgus overcorrection. Overstuffing the medial compartment with a thick insert forces the mechanical axis into neutral or valgus, transferring pathological compressive loads to the unreplaced lateral compartment. This accelerates lateral cartilage breakdown, leading to rapid secondary lateral osteoarthritis. Under manual valgus stress applied in 20° to 30° of knee flexion, the medial compartment must demonstrate 1 to 2 mm of physiological joint opening (“spring” or lift-off). If the medial compartment cannot be distracted by 1 mm under gentle valgus stress, the joint is overstuffed and a thinner bearing or additional bone resection is required. The postoperative mechanical axis should reproduce the patient’s constitutional premorbid alignment, typically 2° to 4° of mechanical varus.

Component Trialing and Kinematic Tracking

With trial components seated, the knee is cycled through a complete passive arc of motion from 0° extension to 120° flexion:

• Full Extension Verification: The knee must achieve complete passive extension matching the contralateral limb without an elastic bounce or extensor lag.
• Mobile-Bearing Kinematics: In mobile-bearing systems, the meniscal bearing must glide smoothly on the polished tibial tray without impingement against the tibial spine wall, posterior capsule, or retained osteophytes. The bearing must not lift off, cock up, or demonstrate paradoxical spinout during deep flexion or axial rotation.
• Fixed-Bearing Kinematics: In fixed-bearing systems, femoral tracking must remain centered on the polyethylene articulating surface without edge loading or coronal liftoff.
• Patellofemoral Clearance: Confirm that the anterior flange of the femoral component does not contact the medial patellar facet or trochlea in deep flexion.

Cement Pressurization and Meticulous Removal of Posterior Extrusion

Cancellous bone preparation dictates cement mantle durability. Lavage the resected tibial and femoral bone surfaces aggressively using pulsed saline lavage to remove all marrow fat, free lipids, and microtrabecular bone debris. Dry the bone beds thoroughly using clean laparotomy sponges and continuous suction. Drill multiple 2.0 mm micro-anchoring holes across the sclerotic cancellous bone bed of the tibia and femur to enhance micromechanical cement interlock. Mix high-viscosity polymethylmethacrylate (PMMA) bone cement and apply it in its early doughy phase to both the implant bonding surfaces and prepared bone beds. Seat and impact the tibial baseplate first, followed by the femoral component. Insert a trial bearing and position the knee in approximately 30° to 45° of flexion under gentle axial compression to maintain uniform cement pressurization while curing. Debride all extruded excess cement meticulously from the anterior, medial, and crucially posterior margins using angled curettes and micro-elevators. Retained posterior cement pearls act as abrasive third-body debris, cause painful capsular impingement, and represent the leading cause of mobile-bearing jamming and bearing dislocation.

10. Pearls and Pitfalls

Adherence to established technical pearls and avoidance of common pitfalls ensures durable implant survivorship and reproducible kinematics.

Surgical Phase / Domain	Master Class Pearl (Best Practice)	Common Pitfall / Error	Rescue & Mitigation Strategy
Patient Selection	Confirm full-thickness medial eburnation and pristine lateral cartilage on 45° Rosenberg and valgus stress views.	Operating on partial-thickness chondromalacia or overlooked lateral compartment chondrolysis.	Perform mandatory direct arthrotomy inspection of ACL, lateral compartment, and patellofemoral joint; convert to TKA if diseased.
Surgical Exposure	Subluxate the patella laterally with a Z-retractor; maintain knee at 90° flexion with hanging leg setup.	Everting the patella, creating severe extensor mechanism stretch, patellar tendon avulsion, or tearing the medial flap.	Release lateral patellar retinaculum internally if exposure is tight; never force patellar eversion.
Osteophyte Debridement	Thoroughly excise all marginal medial and intercondylar notch osteophytes before making bone cuts.	Leaving intercondylar osteophytes that impinge on the ACL, or medial osteophytes that tent the MCL.	Use narrow curved rongeurs and motorized burrs; inspect the notch to confirm free ACL excursion through full ROM.
Tibial Resection	Replicate native posterior tibial slope (5° to 7°) and resect 2 to 4 mm below the deepest medial defect.	Cutting excessive slope (> 8°) or under-resecting bone, leading to a tight joint and flexion contracture.	Re-pin cutting block and recut tibia if slope is aberrant; verify minimum composite implant height with spacer block.
Sagittal Tibial Cut	Position cut 1 to 2 mm medial to medial tibial spine, parallel to AP axis, stopping exactly at horizontal plane.	Overcutting downward past the horizontal cut into the anterior cortex, creating a fracture stress riser.	Use a fine reciprocating saw with depth markings; check junction with a dental pick; ensure wafer lifts without force.
Femoral Preparation	Ensure femoral component sits flush with anterior condylar cartilage without anterior ledge or overhang.	Anterior overhang of femoral component into trochlear groove causing painful patellofemoral impingement.	Recess femoral component or downsize femoral component; contour anterior condylar bone margin with a fine rasp.
Gap Balancing & Alignment	Maintain slight constitutional undercorrection (2° to 4° varus); confirm 1 to 2 mm joint spring under valgus stress.	Overstuffing the joint into mechanical neutral or valgus, transferring load to the unreplaced lateral compartment.	Downsize polyethylene bearing thickness; recut distal femur or increase tibial slope if extension or flexion gap is tight.
Cementation & Cleanup	Pulsed lavage of cancellous bone; drill multiple 2.0 mm micro-holes; meticulously debride posterior cement.	Leaving extruded cement pearls in the posterior condylar recess, causing bearing impingement or dislocation.	Pass a curved micro-curette and dental mirror along posterior condylar and posterior tibial margins before PMMA sets.

11. Complications & Avoidance Strategies

Complications following medial unicompartmental knee replacement can be prevented through rigorous patient selection, precise bone resection, and meticulous soft-tissue preservation.

Complication	Reported Incidence / Risk Factors	Pathophysiologic Mechanism	Intraoperative Prevention & Avoidance Strategy	Diagnostic Workup & Management Algorithm
Periprosthetic Tibial Plateau Fracture	0.5% to 2.5%; female gender, osteopenia, sagittal saw overcutting, aggressive component impaction.	Stress concentration from vertical saw cut extending past horizontal cut, creating a notch in the tibial metaphysis.	Use reciprocating saw for sagittal cut; stop precisely at horizontal plane; avoid lateralizing cut into tibial spine.	Plain radiographs and CT scan. Nondisplaced: protected weight-bearing. Displaced: open reduction internal fixation or revision to TKA with stem.
Mobile Bearing Dislocation	0.5% to 1.5% in mobile-bearing designs; gap imbalance, retained posterior cement, trauma, ligament laxity.	Flexion-extension gap mismatch, excessive posterior tibial slope, retained posterior cement, or deep flexion spinout.	Ensure equal rectangular gaps; restore 5° to 7° slope; clear all posterior cement; confirm 1 mm valgus spring.	Physical exam, stress views, CT. Rule out infection and ligament rupture. Closed reduction trial; revise bearing size or convert to TKA if recurrent.
Lateral Compartment Arthritis Progression	1.0% to 4.0% at 10 years; overcorrection into valgus, unrecognized pre-existing lateral cartilage wear.	Excessive mechanical axis overcorrection shifts weight-bearing contact stresses to unreplaced lateral compartment.	Never correct into valgus; maintain 2° to 4° constitutional varus; confirm normal lateral cartilage on preoperative valgus views.	Standing Rosenberg and full-length alignment views. Conversion to total knee arthroplasty with lateral resurfacing.
Aseptic Loosening (Tibial or Femoral)	1.5% to 3.5% at 10 years; inadequate cement pressurization, sclerotic bone beds, ACL deficiency, obesity.	Micromotion and fibrous membrane formation at bone-cement interface due to inadequate interlock or eccentric loads.	Pulsed lavage of cancellous bone; drill multiple 2.0 mm micro-holes; avoid excessive slope; verify intact ACL.	Serial radiographs (radiolucent lines > 2 mm, progressive tilting). Revision to total knee arthroplasty using primary or augments.
Patellofemoral Joint Impingement	1.0% to 3.0%; anterior femoral component overhang, unaddressed lateral facet arthrosis.	Protrusion of anterior femoral component flange impinges on patellar facet or trochlear groove during flexion.	Size femoral component accurately; ensure anterior condylar transition is completely flush; bevel bone edges.	Axial Merchant radiographs and clinical palpation. Arthroscopic debridement or conversion to TKA if intractable.
Periprosthetic Joint Infection (PJI)	0.2% to 0.6%; diabetes mellitus, immunosuppression, prolonged operative time, wound drainage.	Bacterial colonization and biofilm formation on prosthetic components and polymethylmethacrylate cement.	Standard prophylactic intravenous antibiotics; laminar flow; meticulous sterile technique; watertight closure.	Serum ESR/CRP, diagnostic joint aspiration (cell count, differential, culture, alpha-defensin). DAIR for acute; revision for chronic.

12. Postoperative Care & Rehabilitation

Preservation of the quadriceps mechanism, patellar ligamentous structures, and cruciate ligaments permits accelerated postoperative functional recovery compared to total knee arthroplasty.

Immediate Postoperative Management & Fast-Track Pathways

• Weight-Bearing Status: Immediate full weight-bearing as tolerated (FWBAT) is initiated on postoperative day 0, utilizing crutches or a walker for balance assistance.
• Multimodal Analgesia: Preemptive oral analgesics (acetaminophen, celecoxib, gabapentin), intraoperative local infiltration analgesia (LIA with ropivacaine, epinephrine, and ketorolac), and an ultrasound-guided adductor canal block provide superior pain control without motor blockade.
• Same-Day Discharge Criteria: Medial UKA is highly suited for outpatient surgery. Discharge criteria include stable vital signs, independent ambulation of at least 100 feet, independent stair ascent and descent, adequate bladder emptying, and tolerable pain controlled with oral medications.
• Venous Thromboembolism (VTE) Prophylaxis: Standard-risk patients receive aspirin 81 mg orally twice daily for 4 to 6 weeks, combined with early aggressive mobilization and mechanical compression devices. High-risk patients receive risk-stratified direct oral anticoagulants or low-molecular-weight heparin.

Phase-by-Phase Rehabilitation Protocol

• Phase I: Protection and Activation (Weeks 0 to 2): Immediate active-assisted range of motion aiming for 0° to 90° of flexion within the first 72 hours. Quadriceps reactivation through isometric quadriceps sets, straight leg raises, and ankle pumps. Cryotherapy and elevation for edema control. Transition from walker/crutches to a single cane as quadriceps control recovers, typically by postoperative day 7 to 10.
• Phase II: Range of Motion and Functional Strengthening (Weeks 2 to 6): Progression of active knee flexion to 120° or greater. Discontinue all assistive walking devices once normal gait kinematics are established. Initiate closed-chain kinetic strengthening, including mini-squats (0° to 45°), leg press with light resistance, step-ups, and stationary cycling with minimal resistance. Proprioceptive balance exercises on stable surfaces.
• Phase III: Progressive Resistance and Endurance (Weeks 6 to 12): Advanced closed-chain strengthening, hamstring curls, eccentric quadriceps training, and progressive resistance stationary cycling. Low-impact aerobic conditioning, including outdoor walking and swimming (flutter kick once incisions are healed). Proprioceptive training on unstable surfaces (BOSU ball, wobble board).
• Phase IV: Unrestricted Activity and Return to Recreation (> 12 Weeks): Maintenance strengthening and flexibility exercises. Gradual return to low-to-moderate impact recreational sports, such as golf, doubles tennis, brisk hiking, cycling, and downhill skiing for proficient skiers. High-impact contact sports are generally discouraged to preserve long-term implant fixation.

13. Evidence-Based Synthesis

Modern clinical decision-making is grounded in high-quality randomized controlled trials and prospective registry analyses, corroborated by seminal biomechanical and anatomical descriptions.

Clinical Question / Investigation	Level of Evidence	Pivotal Landmark Study	Study Design & Methodology	Cohort Size / Follow-up	Primary Outcome / Effect Size	DOI
Functional recovery and early outcomes of UKA versus TKA in medial compartment OA	Level I (Therapeutic)	Pongcharoen B, Liengwattanakol P, Boontanapibul K (2023)	Randomized Controlled Trial	100 patients (50 UKA vs 50 TKA)	Faster early functional recovery, greater active knee flexion (128° vs 115° at 6 weeks, p < 0.001), and shorter hospital stay in the UKA cohort.	DOI
Clinical outcomes and complications of UKA versus HTO for isolated medial compartment OA	Level I (Therapeutic)	Zhang B, Qian H, Wu H et al (2023)	Systematic Review / Meta-Analysis	22 comparative studies (3,842 knees)	UKA demonstrated significantly lower complication rates (OR 0.54, p=0.002), faster functional rehabilitation, and lower early revision rate compared to HTO.	DOI
Impact of immersive virtual reality simulation on surgical accuracy and proficiency in UKA	Level I (Therapeutic)	McKinney B, Dbeis A, Lamb A et al (2022)	Randomized Controlled Trial	24 orthopaedic residents (randomized trial)	VR-trained surgeons demonstrated significantly fewer procedural errors (p = 0.004) and superior objective technical performance scores during UKA.	DOI
Long-term implant survivorship and biomechanics of mobile-bearing medial UKA	Level IV (Prognostic / Case Series)	David W. Murray, John Goodfellow, John O’Connor (1998)	Seminal Conceptual / Observational Study	143 knees, up to 15-year follow-up	98% 10-year and 95% 15-year cumulative implant survival in anteromedial osteoarthritis, confirming durability of mobile-bearing design.	DOI
Comparative perioperative mortality, medical morbidity, and revision rates between UKA and TKA	Level IV (Prognostic / Case Series)	Alexander D. Liddle, Andrew Judge, Hemant Pandit et al (2014)	Seminal Conceptual / Observational Study	101,330 propensity-matched patients (NJR)	UKA showed substantially lower 30-day mortality (hazard ratio 0.29), fewer thromboembolic events, but higher 8-year cumulative revision rates than TKA.	DOI

14. References

• Pongcharoen B, Liengwattanakol P, Boontanapibul K. Comparison of Functional Recovery Between Unicompartmental and Total Knee Arthroplasty: A Randomized Controlled Trial. J Bone Joint Surg Am. 2023. doi:10.2106/JBJS.21.00950 [Level I (Therapeutic): Randomized Controlled Trial]
• Zhang B, Qian H, Wu H et al. Unicompartmental knee arthroplasty versus high tibial osteotomy for medial knee osteoarthritis: A systematic review and meta-analysis. J Orthop Surg (Hong Kong). 2023. doi:10.1177/10225536231162829 [Level I (Therapeutic): Systematic Review / Meta-Analysis]
• McKinney B, Dbeis A, Lamb A et al. Virtual Reality Training in Unicompartmental Knee Arthroplasty: A Randomized, Blinded Trial. J Surg Educ. 2022. doi:10.1016/j.jsurg.2022.06.008 [Level I (Therapeutic): Randomized Controlled Trial]
• David W. Murray, John Goodfellow, John O’Connor. The Oxford medial unicompartmental arthroplasty. Journal of Bone and Joint Surgery – British Volume. 1998. doi:10.1302/0301-620x.80b6.8177 [Level IV (Prognostic / Case Series): Seminal Conceptual / Observational Study]
• Alexander D. Liddle, Andrew Judge, Hemant Pandit et al. Adverse outcomes after total and unicompartmental knee replacement in 101 330 matched patients: a study of data from the National Joint Registry for England and Wales. The Lancet. 2014. doi:10.1016/s0140-6736(14)60419-0 [Level IV (Prognostic / Case Series): Seminal Conceptual / Observational Study]

15. Master Class Pro-Tip

The definitive secret to long-term survivorship and natural kinematics in medial unicompartmental knee replacement lies in respecting native joint laxity: never aim for neutral mechanical alignment, and never release the medial collateral ligament. Resect bone to match the implants, debride all peripheral and intercondylar notch osteophytes to unfurl the contracted capsule, and verify that the medial compartment retains 1 to 2 mm of physiologic ‘spring’ under valgus stress in 20° to 30° of flexion. Leaving the knee in 2° to 4° of constitutional varus protects the lateral compartment from devastating stress overload and preserves the natural biomechanics of the unreplaced cruciate and collateral linkages.

Master Class Surgical Pearl: Medial Unicondylar Knee Arthroplasty Gap Balancing and Alignment