Bicompartmental Knee Replacement

1. High-Yield Summary

Bicompartmental knee arthroplasty (BKA) is a joint-preserving, compartment-specific reconstruction designed for patients presenting with combined, end-stage osteoarthritis involving two of the three compartments of the knee joint. In the vast majority of clinical scenarios, this entails combined medial tibiofemoral and patellofemoral compartment degeneration, while the lateral tibiofemoral compartment and cruciate ligament architecture remain structurally intact.

• Primary Clinical Indication: End-stage bicompartmental (medial and patellofemoral) osteoarthritis refractory to conservative care in a physiologically active patient with intact, functional cruciate ligaments and an unaffected lateral compartment.
• The Critical Procedural Move: Flange-runner alignment. The transition zone between the distal flange of the trochlear component and the anterior margin of the unicondylar femoral component must be coplanar or slightly recessed anteriorly. Any proud ridge will cause patellar catching, crepitus, or patellar clunk during mid-flexion.
• Catastrophic Error to Avoid: Performing BKA in the setting of occult anterior cruciate ligament (ACL) insufficiency or fixed varus deformity exceeding 10 degrees. Unrecognized ACL deficiency leads to rapid eccentric loading, anterior tibial translation, and catastrophic early component loosening.
• Kinematic Advantage: By preserving the native anterior and posterior cruciate ligaments, the knee retains natural proprioception, physiological femoral rollback, and normal screw-home kinematics, resulting in superior Forgotten Joint Scores (FJS) compared to standard total knee arthroplasty (TKA).

2. Indications and Patient Selection

Patient selection is the primary determinant of mid-term and long-term survivorship in bicompartmental arthroplasty. Unlike tricompartmental total knee arthroplasty, BKA relies entirely on the mechanical integrity of the retained native structures.

Clinical Category	Patient Criteria & Anatomic Benchmarks	Clinical Rationale
Absolute Indications	Concurrent Kellgren-Lawrence Grade 3 or 4 osteoarthritis in the medial and patellofemoral compartments with failed conservative therapy.	Focal disease amenable to isolated joint compartment resurfacing.
Cruciate Status	Functionally intact Anterior Cruciate Ligament (ACL) and Posterior Cruciate Ligament (PCL).	Mandatory for anteroposterior stability and normal kinematics; absent ACL is an absolute contraindication.
Alignment Constraints	Varus mechanical axis deformity < 10 degrees, fully correctable to neutral on manual stress examination.	Fixed coronal deformity indicates contracture or collateral attenuation requiring extensive soft-tissue release incompatible with BKA.
Motion Criteria	Flexion arc > 90 degrees with flexion contracture < 10 degrees.	Severe pre-existing contractures cannot be reliably corrected without sacrificing cruciate ligaments.
Contraindications	Inflammatory arthritis (rheumatoid, psoriatic), active joint infection, lateral compartment chondral loss (Outerbridge > 2), or prior lateral meniscectomy.	Systemic disease or existing lateral wear causes early progressive failure in unresurfaced regions.

3. Preoperative Planning & Biomechanical Criteria

Imaging Review

A rigorous preoperative radiographic workup is essential to evaluate mechanical alignment, isolate degenerative zones, and confirm preservation of the uninvolved compartment:

• Full-Length Standing Hip-to-Ankle View: Determines mechanical axis deviation, mechanical lateral distal femoral angle (mLDFA), and medial proximal tibial angle (MPTA). Confirms that varus malalignment does not exceed 10 degrees.
• Rosenberg View (45-degree PA Weight-Bearing): Unmasks posterior joint space narrowing in the medial compartment that may be concealed on standard full-extension radiographs. Evaluates the lateral compartment joint space under load.
• True Lateral View (30 degrees flexion): Evaluates patellar height (Insall-Salvati and Caton-Deschamps indices), anterior femoral condylar offset, and posterior tibial slope.
• Merchant or Skyline Axial View: Quantifies patellofemoral joint space loss, lateral patellar tilt, and subluxation (Iwano classification).
• Magnetic Resonance Imaging (MRI): Strongly recommended when clinical cruciate integrity is ambiguous. Accurately verifies intact ACL and PCL fibers, lateral meniscus morphology, and full-thickness lateral compartment cartilage retention.

Equipment Checklist & Implant Systems

Operating rooms must be prepared with modular unlinked bicompartmental systems or contemporary patient-matched monolithic configurations:

• Unicondylar Knee Arthroplasty (UKA) instrumentation: Extramedullary and intramedullary tibial alignment guides, distal femoral resection blocks, and milling or burring instruments.
• Patellofemoral Arthroplasty (PFA) instrumentation: Trochlear cutting jigs, anterior femoral sizing guides, and patellar resurfacing clamps.
• Specialized retractors: Hohmann retractors, Z-retractors for patellar eversion or subluxation, and collateral ligament protective shields.
• High-speed surgical burr and sagittal saw blades with dedicated submillimeter calibration.
• Radiopaque polymethylmethacrylate (PMMA) bone cement and pulsatile lavage system with vacuum mixing setup.

Examination Under Anesthesia (EUA)

Prior to surgical incision, perform an exhaustive physical stability and alignment assessment under full muscular relaxation. Confirm passive reducibility of varus deformity to neutral alignment with valgus stress applied at 20 degrees of flexion. Verify a firm Lachman endpoint and negative pivot-shift test. Any subtle anterolateral rotatory instability or soft Lachman endpoint necessitates intraoperative cruciate direct inspection and immediate readiness to convert to total knee arthroplasty.

4. Patient Positioning & Surgical Setup

Setup and Leg Position

The patient is placed in the standard supine position on a radiolucent operating table. A pneumatic tourniquet is positioned around the upper thigh. A lateral thigh post or mechanical leg holder is positioned to permit unrestricted knee flexion from 0 to at least 120 degrees while providing stable counter-resistance during valgus stressing of the knee during medial compartment exposure. A foot bolster is secured to the table to maintain 90 degrees of knee flexion without manual holding.

Tourniquet and Safety Corridors

Pad all pressure points meticulously, particularly the contralateral peroneal nerve corridor and sacral region. Exsanguinate the operative limb with an Esmarch bandage and inflate the tourniquet to 200 to 250 mmHg (or 100 mmHg above systolic arterial pressure). Ensure wide sterile preparation extending from the tourniquet edge to the mid-calf.

5. Surgical Approach & Internervous Plane

Surgical Approach

A straight midline anterior longitudinal skin incision is performed, beginning approximately 4 cm proximal to the superior pole of the patella and terminating 2 cm distal to the tibial tubercle. Full-thickness fasciocutaneous flaps are developed to minimize skin edge necrosis.

Deep exposure is achieved via a standard medial parapatellar arthrotomy. The capsular incision extends proximally through a 2 to 3 cm cuff of the rectus femoris and vastus medialis tendon junction, curves around the medial border of the patella preserving a 3 mm soft-tissue sleeve on the patella for secure repair, and continues distally along the medial border of the patellar tendon to the medial tibial plateau.

Internervous & Intermuscular Plane

An anterior midline knee approach traverses no true internervous plane. The quadriceps musculature is supplied proximally by the femoral nerve (L2-L4). Distally, the infrapatellar branches of the saphenous nerve cross horizontally over the anterior joint line. Careful subcutaneous dissection prevents iatrogenic transection of these branches, minimizing painful postoperative neuroma formation and anterior hypesthesia.

6. Step-by-Step Surgical Technique

Step 1: Exposure, Cartilage Inspection, and Cruciate Assessment

Incise the synovium and subluxate or evert the patella laterally. Excise the retropatellar fat pad selectively to expose the anterior horn of the medial meniscus and anterior tibial plateau while preserving the patellar tendon blood supply. Immediately inspect the anterior cruciate ligament. Verify that the ACL is structurally intact, well-vascularized, and exhibits normal tension when hooked with a probe. Inspect the lateral tibiofemoral compartment: the lateral femoral condyle and lateral tibial plateau must exhibit intact, undamaged hyaline cartilage without eburnation or deep fissuring. If the ACL is ruptured or the lateral compartment demonstrates Outerbridge Grade 3 or 4 wear, abort BKA and proceed directly to standard total knee arthroplasty.

Step 2: Unicondylar Tibial and Distal Femoral Resection

Position the extramedullary tibial cutting guide parallel to the mechanical axis of the tibia in the coronal plane, setting posterior tibial slope to match the patient’s native anatomy (typically 3 to 5 degrees). Protect the medial collateral ligament with a curved retractor and safeguard the ACL tibial insertion with a vertical reciprocating saw blade guard. Perform the horizontal proximal medial tibial cut, aiming for a conservative 2 to 4 mm resection below the deepest portion of the medial defect, preserving strong subchondral bone. Execute the vertical sagital cut strictly adjacent to the medial tibial spine. Resect the distal medial femoral condyle perpendicular to the mechanical axis using the femoral resection block, ensuring conservative bone removal matching the implant thickness.

Step 3: Trochlear Resection and Patellofemoral Preparation

Align the anterior trochlear cutting guide flush with the anterior cortex of the femur to avoid anterior notching or overstuffing the patellofemoral joint. Resect the dysplastic or arthritic trochlear groove, referencing the epicondylar axis and Whiteside’s line to set 3 to 5 degrees of external rotation. Measure patellar thickness with a digital caliper. Perform planar patellar resection parallel to the anterior cortical plane, removing bone equivalent to the thickness of the prospective patellar component (typically 8 to 10 mm) while maintaining at least 12 to 14 mm of residual bone stock. Drill anchor peg holes in the patella using the drill guide.

Step 4: Component Trialing, Kinematic Tracking, and Gap Balancing

Insert trial components for the medial UKA (femoral runner, tibial tray, and polyethylene insert) and PFA (trochlear flange and patellar button). Assess flexion and extension gaps in the medial compartment: gaps must be symmetric with 1 to 2 mm of physiologic laxity under valgus stress in mid-flexion. Evaluate patellar tracking through full range of motion from 0 to 120 degrees using the “no-thumb” technique. The patella must track centrally within the prosthetic trochlear groove without tilting, snapping, or subluxating.

Step 5: Cementation Technique and Final Implantation

Cleanse all prepared trabecular bone beds thoroughly using high-pressure pulsatile lavage with saline, and dry the bone surfaces completely with suction and dry sponges. Mix high-viscosity PMMA bone cement under vacuum. Pressurize cement into the cancellous bone of the proximal medial tibia, distal medial femur, anterior femoral trochlea, and patellar peg holes. Seat the final implants sequentially: tibial component first, followed by the unicondylar femoral component, trochlear component, and patellar button. Impact firmly with designated seating tools. Meticulously excise all extruded cement flash around the margins, paying special attention to the posterior condyle and the interface between the two femoral components.

Critical Move: Flange-Runner Transition Zone

The single most technically demanding step in modular bicompartmental arthroplasty is managing the mechanical junction between the anterior trochlear component and the distal unicondylar femoral runner. These two components must create a harmonious, uninterrupted trajectory. If the distal edge of the trochlea is proud relative to the UKA runner, it acts as a mechanical ledge, precipitating patellar clunk or rapid polyethylene wear. Verify with a flat tactile probe that the junction is completely coplanar or that the trochlea is seated 0.5 mm recessed relative to the condylar runner.

7. Pearls and Pitfalls

Procedural Stage	Master Class Pearls	Technical Pitfalls & Rescue Maneuvers
Patient Selection	Confirm cruciate integrity with both MRI and direct intraoperative digital palpation before making any bone cuts.	Overlooking subtle ACL attenuation leads to early anterior tibial subluxation and catastrophic tibial tray loosening.
Tibial Resection	Preserve the deep medial collateral ligament by maintaining the saw blade within the protective cutting block corridor; respect native 3 to 5 degree posterior slope.	Over-resecting the tibia (> 5 mm) breaches strong cortical bone, causing implant subsidence; inadvertent MCL transection requires immediate conversion to TKA.
Trochlear Sizing	Size the trochlear shield based on mediolateral width to avoid lateral condylar overhang; ensure 3 degrees of external rotation.	Internal rotation of the trochlear component causes patellar maltracking, lateral tilt, and persistent anterior knee pain.
Flange-Runner Junction	Trial both components simultaneously. Use a high-speed burr on the intercondylar ridge if bone prominence prevents flush seating.	Leaving a step-off between the trochlear component and the UKA runner induces mechanical catching during deep knee flexion.
Cement Cleanout	Maintain knee in 30 degrees of flexion during cement curing and inspect the posterior capsule with a curved mirror.	Retained posterior cement nodules cause mechanical impingement, flexion loss, and abrasive third-body polyethylene wear.

8. Postoperative Care & Rehabilitation Protocol

Immediate Postoperative Protocol

Patients are mobilized on the day of surgery (Postoperative Day 0) with full weight-bearing as tolerated (WBAT) under physical therapy guidance. A multimodal opioid-sparing analgesia regimen combining local infiltration analgesia (LIA), adductor canal peripheral nerve blockade, and scheduled acetaminophen and NSAIDs is initiated. Chemical deep vein thrombosis (DVT) thromboprophylaxis is administered in accordance with standard AAOS guidelines (aspirin 81 mg twice daily for 4 to 6 weeks in standard-risk patients).

Phased Rehabilitation Protocol

• Phase I: Early Protection & Activation (Weeks 0 to 2): Immediate full weight-bearing with crutches or walker; active and active-assisted range of motion focusing on full passive extension and quad sets; straight leg raises without lag; cryotherapy.
• Phase II: Range of Motion & Neuromuscular Control (Weeks 2 to 6): Wean assistive devices; achieve flexion > 110 degrees; stationary cycling with low resistance; closed-chain kinetic exercises (mini-squats, leg press).
• Phase III: Progressive Resistance & Proprioception (Weeks 6 to 12): Eccentric quadriceps strengthening, hamstring activation, balance and proprioceptive board training, elliptical trainer.
• Phase IV: Functional Conditioning & Return to Activity (> 12 Weeks): Low-impact aerobic conditioning, golf, cycling, swimming, and hiking. High-impact contact sports are discouraged to protect implant longevity.

9. Clinical Outcomes & Functional Kinematics

Because bicompartmental arthroplasty preserves both the anterior and posterior cruciate ligaments, normal knee kinematics are maintained. During flexion, the normal femoral rollback occurs naturally across the lateral tibiofemoral articulation, while the medial condyle serves as a stable pivot center. In vivo kinematic gait analyses demonstrate near-normal quadriceps mechanics and physiological knee flexion during stair ascent and descent.

Validated patient-reported outcome measures (PROMs) consistently demonstrate superior Knee Society Functional Scores (KSS), Oxford Knee Scores (OKS), and Forgotten Joint Scores (FJS-12) compared to cohort-matched total knee arthroplasty. Patients frequently report a “natural feeling knee” with accelerated recovery and less postoperative pain.

10. Complications & Revision Strategies

Complications Matrix

Complication	Incidence	Etiology & Risk Factors	Management & Prevention
Lateral Compartment OA Progression	3% to 8% at 10 years	Pre-existing occult lateral chondromalacia, coronal overcorrection into valgus alignment.	Maintain neutral mechanical alignment; avoid overstuffing medial joint; convert to TKA if symptomatic.
Patellofemoral Maltracking	2% to 5%	Trochlear component internal rotation, lateral facet overhanging, improper patellar button placement.	Set trochlear rotation to 3-5 degrees external; lateral retinacular release; re-revision of patellar button.
Aseptic Loosening	1% to 3%	Poor cement technique, inadequate bone interdigitation, excessive tibial slope, or ACL loss.	Vacuum cement pressurization, pulsed lavage drying; revision to modular TKA with stems if necessary.
Arthrofibrosis	< 2%	Inadequate pain control, delayed rehabilitation, complex regional pain syndrome.	Aggressive early physical therapy; manipulation under anesthesia (MUA) within 6 to 8 weeks if flexion < 90 degrees.
Periprosthetic Infection	< 1%	Hematogenous seeding, wound healing complications, diabetes, immunosuppression.	Parenteral antibiotic prophylaxis, sterile technique; DAIR or two-stage revision protocol.

Revision to Total Knee Arthroplasty

When bicompartmental knee arthroplasty fails, revision to a primary or revision total knee arthroplasty is standard. Bone loss is typically modest compared to failed TKA, as distal femoral bone resection is minimal and bone stock in the uninvolved lateral compartment remains intact. In most instances, revision can be accomplished using standard primary TKA implants or modular augments with short stem extensions if medial tibial bone loss is encountered.

11. Modular vs. Monolithic Arthroplasty

The evolution of bicompartmental arthroplasty spans two distinct engineering designs: first-generation monolithic implants and contemporary modular systems. First-generation monolithic designs utilized a single, continuous femoral component that incorporated both the anterior trochlea and the medial condyle. While intuitively attractive, monolithic implants suffered from high early revision rates (up to 15% at 5 years) due to severe anatomical mismatches, restricted component sizing options, and inability to independently optimize rotation and alignment for both the trochlear groove and the distal condyle.

Contemporary practice has shifted almost universally toward modular bicompartmental arthroplasty, utilizing independent, unlinked unicondylar and patellofemoral components. Modular systems grant the operating surgeon complete independence in selecting component sizing, establishing condylar joint line position, and orienting trochlear rotation, resulting in dramatically improved survivorship and lower revision rates.

12. Surgical Video & Faculty Commentary

In the accompanying operative presentation, Dr. Ormonde Mahoney demonstrates the clinical rationale and operative execution of bicompartmental knee replacement. The video reviews patient selection guidelines, highlights exposure through a medial parapatellar arthrotomy, demonstrates precision bone cuts for the medial compartment, and walks through the meticulous trialing process required to balance the patellofemoral and medial tibiofemoral joints while preserving the native cruciate ligaments.

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
Does BKA provide superior clinical recovery and ROM compared to TKA in bicompartmental OA?	Level I	Ma JX, He WW, Kuang MJ et al (2017)	Systematic Review / Meta-Analysis	10 comparative studies (468 BKAs vs 512 TKAs)	BKA achieved significantly greater postoperative flexion (WMD 5.43°, p=0.003) and reduced blood loss (WMD -188.4 mL, p<0.001), with equivalent Knee Society Scores and complication rates.	DOI
How do updated functional outcomes, kinematics, and revision rates compare between BKA and TKA?	Level I	Zhang R, Shen X, Yan K et al (2025)	Systematic Review / Meta-Analysis	16 studies (1,245 patients; mean follow-up 4.8 years)	BKA demonstrated higher Forgotten Joint Scores, superior physiological knee kinematics, and earlier return to activity, balanced against a slightly higher cumulative revision rate in monolithic implants.	DOI
What are the mid-to-long-term survivorship and functional outcomes of modular BKA for medio-patellofemoral OA?	Level I	Amit P, Singh N, Soni A et al (2020)	Systematic Review / Meta-Analysis	9 studies (331 patients / 341 knees; follow-up to 17 years)	Modular BKA demonstrated comparable KSS and KOOS to TKA with better ROM and ‘forgotten knee’ status; implant survivorship was 95.1% at 5 years and 58% at 17 years.	DOI

14. References

• Ma JX, He WW, Kuang MJ et al. Efficacy of bicompartmental knee arthroplasty (BKA) for bicompartmental knee osteoarthritis: A meta analysis. Int J Surg. 2017. doi:10.1016/j.ijsu.2017.08.556 [Level I: Systematic Review / Meta-Analysis]
• Zhang R, Shen X, Yan K et al. Comparative efficacy and safety of bicompartmental versus total knee arthroplasty: a systematic review and update meta-analysis. J Orthop Surg Res. 2025. doi:10.1186/s13018-024-05384-6 [Level I: Systematic Review / Meta-Analysis]
• Amit P, Singh N, Soni A et al. Systematic Review of Modular Bicompartmental Knee Arthroplasty for Medio-Patellofemoral Osteoarthritis. J Arthroplasty. 2020. doi:10.1016/j.arth.2019.09.042 [Level I: Systematic Review / Meta-Analysis]

15. Master Class Pro-Tip

When performing modular bicompartmental knee arthroplasty, never balance the patellofemoral and medial compartments in isolation. Prepare the medial unicondylar cuts first to establish coronal alignment and joint line height, but resect the trochlea with strict avoidance of internal rotation and anterior displacement. The critical transition zone between the distal edge of the trochlear flange and the anterior margin of the unicondylar femoral runner must be absolutely flush or slight trochlear anterior recessing; any proud ridge will create patellar catching or patellar clunk syndrome during mid-flexion.

Master Class Surgical Pearl: Flange-Runner Transition & Kinematic Tracking