Patellofemoral Replacement

Archival Presentation Notice: This article modernizes the multimedia presentation “Patellofemoral Arthroplasty” originally presented by Jess H. Lonner, MD (Director, Knee Replacement Surgery, Pennsylvania Hospital). The historical Flash lecture has been reconstructed into the synchronized video slide presentation above, with the original file preserved in the digital archives (Patellofemoral_Replacement_Lonner.swf).

1. High-Yield Summary

Patellofemoral arthroplasty (PFA) is a targeted partial knee reconstruction designed to replace the articulating surfaces of the anterior femoral trochlea and the patella while preserving the native tibiofemoral compartments, menisci, and cruciate ligaments. When executed with strict adherence to patient selection and component alignment, PFA delivers rapid functional recovery, superior knee kinematics, and physiological extensor mechanism dynamics.

• Primary Clinical Indication: Isolated, end-stage patellofemoral osteoarthritis (Outerbridge Grade IV, Iwano Stage III–IV) or severe trochlear dysplasia refractory to comprehensive nonoperative therapy, in the setting of strictly preserved medial and lateral tibiofemoral compartments and competent cruciate ligaments.
• The Critical Move: Precision rotational alignment of the femoral trochlear component parallel to the surgical transepicondylar axis and perpendicular to Whiteside’s line, combined with establishing a flush, non-proud distal articular transition zone into the intercondylar notch cartilage.
• Catastrophic Technical Errors to Avoid: Trochlear component internal rotation (instigating severe lateral patellar tilt, subluxation, and accelerated polyethylene wear); anterior overstuffing (elevating retinacular tension, restricting terminal flexion, and generating intractable anterior knee pain); and excessive patellar resection violating the 12 mm residual bone bridge threshold (predisposing to catastrophic intraoperative or postoperative patellar fracture).
• Long-Term Durability: Contemporary onlay trochlear prostheses achieve 10-year implant survivorship rates between 88% and 92%, with disease progression in the unresurfaced tibiofemoral compartments representing the primary mode of late revision to total knee arthroplasty (TKA).

2. Indications

Patient selection represents the single most critical determinant of long-term prosthetic survivorship in patellofemoral arthroplasty. Diagnostic evaluation must confirm that degenerative wear is strictly confined to the patellofemoral articulation without occult involvement of the tibiofemoral compartments.

Absolute Indications

• Isolated End-Stage Patellofemoral Osteoarthritis: Outerbridge Grade IV full-thickness chondral loss, Iwano Stage III–IV radiographic disease, or bone-on-bone eburnation strictly localized to the patella and femoral trochlea.
• Post-Traumatic Patellofemoral Arthrosis: Secondary articular incongruity and degenerative chondropathy following patellar fracture or recurrent traumatic patellar subluxation/dislocation, with preserved tibiofemoral joint spaces.
• High-Grade Trochlear Dysplasia with Cartilage Loss: Dejour Type B, C, or D trochlear morphology presenting with severe anterior knee pain, articular breakdown, and recurrent instability refractory to conservative realignment procedures.
• Failed Conservative Management: Persistent, debilitating pain and functional impairment during activities of daily living (stair climbing, rising from a seated position, squatting) despite a minimum of 6 months of structured nonoperative care, including targeted quadriceps/core physical therapy, unloader taping, bracing, non-steroidal anti-inflammatory drugs, and image-guided intra-articular injections.

Relative Indications

• Younger or Physiologically Active Patients (< 60 Years): Candidates desiring bone-preserving reconstruction, physiological joint proprioception, and enhanced active knee flexion compared to conventional total knee arthroplasty.
• Sedentary or Elderly Patients with Isolated Disease: Low-demand elderly individuals with severe anterior knee pain who are candidates for rapid-recovery, outpatient arthroplasty with minimal perioperative blood loss.
• Extensor Mechanism Malalignment Amenable to Realignment: Patients with elevated tibial tubercle-trochlear groove (TT-TG) distance (15 to 20 mm) eligible for concurrent or staged tibial tubercle osteotomy (e.g., anteromedialization) to optimize prosthetic tracking.

Contraindications

• Tibiofemoral Osteoarthritis (Grade ≥ 2): Radiographic (Kellgren-Lawrence Grade 2 to 4) or arthroscopic evidence of weight-bearing chondral loss in the medial or lateral tibiofemoral compartments. This is an absolute contraindication, as disease progression necessitates early revision.
• Inflammatory and Systemic Arthropathies: Rheumatoid arthritis, psoriatic arthritis, ankylosing spondylitis, systemic lupus erythematosus, and diffuse crystal arthropathies (gout, calcium pyrophosphate dihydrate deposition disease / chondrocalcinosis) causing pan-articular synovial and cartilage degradation.
• Coronal Plane Malalignment: Fixed varus deformity > 5° or fixed valgus deformity > 5° that cannot be passively corrected to neutral, which produces eccentric overload on the tibiofemoral compartments.
• Cruciate Ligament Deficiency: Chronic rupture or functional insufficiency of the anterior cruciate ligament (ACL) or posterior cruciate ligament (PCL). Unstable tibiofemoral kinematics increase shear stresses across the patellofemoral joint and accelerate compartment degeneration.
• Severe Morbid Obesity (BMI > 40 kg/m²): Substantially elevated contact stresses predispose to premature polyethylene wear, component loosening, and mechanical failure.
• Uncorrectable Extensor Maltracking or Severe Instability: Pathological patella alta (Caton-Deschamps index > 1.3), unaddressed patellar tilt, or habitual dislocations that cannot be corrected with soft-tissue balancing or bony realignment.
• Infection and Soft-Tissue Compromise: Active septic arthritis, osteomyelitis, compromised skin envelope across the anterior knee, or severe neurological deficits affecting the extensor apparatus.
• Significant Range of Motion Loss: Fixed flexion contracture > 10° or overall knee flexion < 90°.

3. Preoperative Planning

Preoperative templating and objective multiplanar imaging are essential to verify anatomical suitability, exclude occult tibiofemoral degeneration, and determine rotational reference axes.

Imaging Review

• Weight-Bearing Anteroposterior (AP) Radiograph: Assesses baseline coronal alignment, joint space symmetry, and presence of marginal osteophytes across the medial and lateral compartments.
• Rosenberg View (45° Flexion PA Weight-Bearing): Mandatory view to detect early posteromedial and posterolateral tibiofemoral joint space narrowing that remains occult on standard extension AP radiographs.
• True Lateral Radiograph at 30° Flexion: Evaluates patellar height using the Caton-Deschamps and Insall-Salvati indices, defines the anterior femoral cortical profile, and screens for trochlear dysplasia (crossing sign, supratrochlear spur, double contour sign per Dejour classification).
• Merchant Axial Skyline View (45° Flexion): Directly evaluates patellofemoral joint space narrowing, subchondral sclerosis, osteophyte distribution, sulcus angle, patellar tilt angle, and lateral subluxation.
• Full-Length Weight-Bearing Hip-to-Ankle Alignment Radiograph: Establishes the mechanical axis of the lower extremity. The mechanical hip-knee-ankle (HKA) angle must fall within 180° ± 3°.
• Computed Tomography (CT) Rotational Profile: Indicated when extensor mechanism maltracking or dysplasia is suspected. Quantifies the tibial tubercle-trochlear groove (TT-TG) distance (normal 20 mm requiring consideration of concurrent tibial tubercle osteotomy), femoral anteversion, and external tibial torsion.
• Magnetic Resonance Imaging (MRI): Utilized when plain radiographs are equivocal to perform high-resolution chondral mapping across the weight-bearing tibiofemoral surfaces and verify structural integrity of the cruciate ligaments and menisci.

Equipment Checklist

• Modern Patellofemoral Replacement System: Contemporary anatomical onlay trochlear components with left/right specific lateral flanges and shallow sulcus geometry, paired with all-polyethylene round or oval three-peg patellar buttons.
• Instrumentation and Guides: Intramedullary and extramedullary femoral alignment guides, anterior femoral referencing cutting blocks, distal chamfer and transition reaming guides, slotted patellar resection guides with depth styluses, and peg drill templates.
• Specialized Surgical Retractors: Curved blunt Hohmann retractors, Z-retractors for anterior femoral exposure, Langenbeck retractors, and double-pronged patellar eversion clamps.
• Measurement and Power Tools: Precision vernier calipers (accurate to 0.5 mm) for measuring native and composite patellar thickness; narrow-kerf sagittal saw blades; high-speed burr for osteophyte debridement.
• Cementing and Irrigation Apparatus: High-pressure pulsatile lavage with brush attachments, vacuum polymethylmethacrylate (PMMA) mixing system, cement injection gun, and silicone-cushioned patellar clamp.
• Backup Arthroplasty Equipment: Sterile total knee arthroplasty instrumentation must be immediately available in the operating room in the event that significant intraoperative tibiofemoral chondropathy is discovered upon arthrotomy.

Examination Under Anesthesia (EUA)

• Passive Range of Motion: Document preoperative terminal extension, flexion limits, and presence of any fixed flexion contractures or hamstring spasticity.
• Collateral Ligament Stability: Perform varus and valgus stress testing at 0° and 30° of knee flexion to confirm firm endpoints and absence of collateral laxity.
• Sagittal Cruciate Stability: Perform the Lachman and anterior drawer tests to verify anterior cruciate ligament competence, and the posterior drawer test at 90° to confirm posterior cruciate ligament stability.
• Patellar Translation and Tilt Testing: Assess medial and lateral patellar glide in extension and 30° flexion (normal translation is 1 to 2 quadrants). Perform the patellar tilt test; an inability to elevate the lateral patellar border to horizontal indicates severe lateral retinacular contracture. Note any dynamic J-sign excursion during passive extension.

4. Patient Positioning & Setup

The patient is placed in the supine position on a radiolucent operating table capable of accommodating intraoperative fluoroscopy or navigation if required. A lateral thigh support is mounted securely to the table frame at the level of the proximal tourniquet, and a distal foot bolster or adjustable triangular leg ramp is positioned to maintain stable knee flexion at 90° while allowing unrestricted passive motion from 0° to 120°.

• Tourniquet Application: A well-padded pneumatic contour tourniquet is positioned around the proximal thigh of the operative limb. The tourniquet can be inflated to 250 mmHg after limb exsanguination, or maintained uninflated until the cementation phase to minimize intraoperative muscle ischemia.
• Padding and Nerve Protection: Contralateral limb bony prominences, including the fibular neck (common peroneal nerve) and calcaneus, are protected with viscoelastic gel pads. A padded roll is placed beneath the contralateral lower extremity.
• Pharmacologic Preparation: Intravenous tranexamic acid (1 to 2 g) is administered 15 to 30 minutes prior to surgical incision to minimize perioperative blood loss and postoperative hematoma formation.

Safety Check

• Motion Corridor Verification: Prior to sterile draping, cycle the knee through full extension to 120° flexion to ensure the limb moves unimpeded without collision against thigh posts, table brackets, or Mayo stands.
• Implant Sizing Verification: Visually inspect that the full sizing continuum for both onlay femoral and all-polyethylene patellar trials and definitive sterile implants are verified in the room.
• Sterile Light and Retractor Clearance: Confirm unhindered surgical sightlines down the intramedullary femoral axis.

5. Approach

A standard anterior midline skin incision measuring approximately 10 to 12 cm is fashioned, centered over the patella and extending from 2 cm proximal to the superior pole distally to the level of the tibial tubercle. Medializing the incision slightly off the anterior prominence of the patella reduces kneeling discomfort postoperatively.

Full-thickness subcutaneous flaps are mobilized with minimal subcutaneous dissection to protect the cutaneous microvasculature. The extensor mechanism is exposed, and a medial parapatellar arthrotomy or midvastus approach is executed based on patient anatomy and soft-tissue mobility:

• Medial Parapatellar Arthrotomy: The capsule is incised longitudinally along the medial border of the quadriceps tendon, leaving a 4 to 5 mm fascial cuff on the medial aspect of the patella for watertight closure. The incision curves gently around the medial patellar facet and extends distally along the medial border of the patellar tendon, terminating medial to the tibial tubercle.
• Midvastus Approach: An alternative muscle-sparing technique in which the vastus medialis obliquus (VMO) muscle belly is split obliquely along its fiber orientation for 1.5 to 2 cm, continuing distally into the medial parapatellar retinaculum. This approach preserves the proximal quadriceps tendon insertion and accelerates early quadriceps rehabilitation.

Internervous Plane

• Innervation Analysis: The medial parapatellar approach does not follow a classical internervous plane between two distinct peripheral nerves. The muscles comprising the anterior extensor apparatus—rectus femoris, vastus lateralis, vastus intermedius, and vastus medialis—are all innervated by muscular branches of the femoral nerve (L2, L3, L4).
• Vastus Medialis Obliquus Protection: In the midvastus approach, the muscle split must remain limited to less than 2.5 cm from the superior patellar border. The principal motor nerve branch entering the vastus medialis traverses the middle-to-proximal third of the muscle; splitting beyond 3 cm risks denervation of the distal VMO fibers, resulting in permanent muscular weakness and secondary extensor maltracking.
• Infrapatellar Branch of the Saphenous Nerve (IPBSN): Courses obliquely across the anteromedial proximal tibia, crossing the patellar tendon distally. Developing the distal flap medial to the tibial tubercle places the IPBSN at risk of direct transection or traction injury, which can cause lateral prepatellar skin numbness or painful neuroma formation. Dissection should remain subperiosteal along the medial tibial border.
• Genicular Arterial Corridors: The superior medial and inferior medial genicular arteries traverse the medial capsular border. Early identification and precise bipolar coagulation prevents postoperative hemarthrosis and subsequent arthrofibrosis.

6. Technique

Patellofemoral replacement demands uncompromising technical accuracy. Every step in bone resection, component rotation, and soft-tissue balancing directly impacts patellar tracking and overall implant survivorship.

Step 1: Exposure and Soft-Tissue Mobilization

• Incise the medial capsule and synovium to gain entry into the knee joint.
• Mobilize the infrapatellar fat pad (Hoffa’s pad) by excising its central portion while preserving lateral and deep soft-tissue attachments to protect the inferior patellar blood supply.
• Sublux or Evert the patella laterally into the lateral gutters with the knee in extension, using a padded two-prong patellar clamp. In muscular or tight extremities, lateral subluxation without forced eversion provides outstanding visualization while mitigating excessive tensile stress on the patellar tendon insertion.
• Inspect the articular cartilage of the medial femoral condyle, lateral femoral condyle, medial tibial plateau, lateral tibial plateau, and menisci systematically. Confirm complete absence of full-thickness Outerbridge Grade III–IV cartilage defects on the weight-bearing tibiofemoral surfaces. If significant tibiofemoral wear is identified, immediately abandon PFA and convert to total knee arthroplasty.
• Debride large peripheral osteophytes from the patella and the medial and lateral trochlear margins using a rongeur and high-speed burr. Debriding marginal patellar osteophytes restores native anatomical dimensions, unmasks true patellar thickness, and permits accurate center-line localization.

Step 2: Bone Preparation / Debridement

• Establish Femoral Reference: Insert the intramedullary alignment rod into the distal femur 5 to 10 mm anterior to the femoral PCL insertion and slightly medial to the center of the intercondylar notch, advancing the rod gently into the femoral diaphysis.
• Position Anterior Cutting Block: Seat the anterior femoral cutting guide flush onto the distal articular surface. Reference the anterior femoral cortex using an anterior cortical feeler stylus to ensure that the resection plane is flush with the anterior shaft, thereby avoiding anterior cortical notching proximally and preventing anterior overstuffing distally.
• Establish Rotational Alignment: Rotate the trochlear cutting guide parallel to the surgical transepicondylar axis (connecting the prominence of the lateral epicondyle to the sulcus of the medial epicondyle) and perpendicular to Whiteside’s anteroposterior trochlear line. The cutting block must be positioned in 3° to 5° of external rotation relative to the posterior condylar axis. Never position the cutting block in internal rotation.
• Perform Anterior Trochlear Resection: Execute the anterior femoral resection using an oscillating saw with a thin-kerf blade held flush against the slotted guide, cooling continuously with cold saline irrigation.
• Prepare Chamfers and Transition Zone: Mount the anterior and distal chamfer cutting blocks. Complete the chamfer bone cuts to match the prosthetic contour. Ream or countersink the distal transition zone so that the distal prosthetic flange transitions seamlessly into the native cartilage of the intercondylar notch.
• Onlay vs Inlay Implant Geometries: Modern onlay designs resurface the entire anterior trochlea down to the distal femoral cuts, elevating the lateral trochlear flange and correcting dysplastic anatomy. Inlay designs require localized inlay recess reaming within the native trochlea; they carry a high incidence of proud component edges, patellar catching, and increased revision rates. Onlay designs are strongly favored.
• Patellar Preparation and Caliper Calibration: Measure the baseline native patellar thickness with precision calipers at its thickest point (normal 22 to 26 mm). Clamp the slotted patellar resection guide parallel to the anterior cortical plane.
• Resect Patellar Facet: Resect the articular surface with an oscillating saw, removing a thickness of bone equivalent to the thickness of the planned prosthetic patellar component (typically 8 to 10 mm). Maintain a mandatory minimum residual bony bridge of 12 to 15 mm. Never resect more than 50% of the total patellar thickness.
• Drill Patellar Peg Holes: Position the patellar drill template slightly medial and superior on the cut bony facet to optimize patellar tracking. Drill the three fixation peg holes using the depth-stop drill bit.

Step 3: Trialing and Provisional Fixation

• Insert Trial Components: Seat the provisional femoral trochlear trial and all-polyethylene patellar trial button.
• Inspect Articular Transition: Palpate the junction between the distal edge of the trochlear trial and the intercondylar notch cartilage. The transition must be completely flush or slightly recessed (0.5 mm). There must be no proud prosthetic lip, ridge, or step-off.
• Perform the “No-Thumb” Technique: Reduce the patella into the prosthetic trochlea without applying any medial digital holding pressure. Cycle the knee through a full range of motion from 0° to 120°.
• Evaluate Kinematic Tracking: Observe patellar engagement into the trochlear groove between 15° and 30° of flexion. The patellar button must engage centrally without lateral tilting, subluxation, or edge-lifting. If the patella tracks smoothly without digital assistance (“Rule of Thumb”), alignment is optimal.
• Verify Composite Thickness: Re-measure the combined thickness of the resected patella and patellar trial using calipers. Total composite thickness must match native pre-resection thickness ± 1 mm. Overstuffing the patellofemoral joint by > 2 mm dramatically elevates extensor retinacular tension, causes flexion loss, and generates persistent postoperative pain.

Step 4: Final Implant Placement or Suture Fixation

• Pulsatile Lavage and Drying: Irrigate the cancellous bone beds of the anterior femur and patella copiously with high-pressure pulsatile saline lavage to clear marrow fat, microscopic bone debris, and blood clots. Dry the bone beds thoroughly with clean laparotomy sponges and high-vacuum suction.
• Cement Application: Mix radiopaque polymethylmethacrylate (PMMA) bone cement under vacuum. During the early doughy phase, pressurize cement into the cancellous beds of the anterior femur and patella using digital pressurization and a cement gun. Apply a uniform 1 to 2 mm mantle onto the back of the femoral component and patellar button.
• Implant Seating: Seat the femoral trochlear prosthesis using the dedicated impactor, driving it flush against the anterior resection bed. Simultaneously seat the all-polyethylene patellar component into the drilled peg holes, compressing it with the silicone-jawed patellar clamp.
• Cement Debridement: Meticulously remove all extruded cement from around the margins of both implants using a curette and freer elevator. Thoroughly inspect the intercondylar notch, suprapatellar pouch, and lateral gutters to ensure no loose cement fragments remain.
• Maintain Polymerization Pressure: Maintain steady compression with the knee flexed to 30°–45° until complete exothermic polymerization is achieved (8 to 10 minutes).
• Assess Final Soft-Tissue Tension: Re-verify patellar tracking using the no-thumb test. If persistent lateral patellar tilt or subluxation is identified, perform a titrated inside-out lateral retinacular release, beginning 1 cm lateral to the patellar border, carefully preserving the lateral superior genicular artery to protect patellar vascularity.
• Watertight Closure: Close the medial parapatellar arthrotomy in layers using interrupted or continuous heavy absorbable #1 or #2 braided sutures, verifying complete watertight soft-tissue apposition with the knee flexed to 30°. Subcutaneous layers are closed with 2-0 and 3-0 absorbable sutures, followed by a subcuticular skin closure and sterile occlusive dressings.

Critical Move

Precision rotational alignment of the femoral trochlear component parallel to the surgical transepicondylar axis and perpendicular to Whiteside’s line is the single most critical, irreversible technical step in patellofemoral replacement. Positioning the component in internal rotation tilts the prosthetic sulcus medially and depresses the lateral trochlear flange, creating an immediate, severe biomechanical vector for lateral patellar tilt, subluxation, edge-loading, and catastrophic premature polyethylene failure.

Equally vital is establishing a seamless, flush articular transition between the distal prosthetic flange and the intercondylar notch cartilage. If the distal edge sits proud by even 1 mm, the inferior pole of the patellar button will impact and catch during terminal flexion (between 70° and 90°), generating severe anterior knee pain, mechanical crepitation, and accelerated component loosening.

7. Pearls and Pitfalls

Adherence to refined surgical nuances safeguards against the most common technical failure modes encountered in patellofemoral replacement.

Pearls

• Set 3° to 5° External Rotation: Always cross-reference Whiteside’s anteroposterior line with the surgical transepicondylar axis to place the trochlear component in 3° to 5° of external rotation, recreating physiological lateral trochlear height.
• Medialize the Patellar Component: Drill peg holes slightly medial and superior on the cut patellar facet. Medialization decreases the effective Q-angle and lateral shear forces, facilitating central patellar engagement into the sulcus without requiring aggressive lateral release.
• Maintain a 12 to 15 mm Bony Bridge: Always measure residual patellar bone thickness with vernier calipers before drilling peg holes. Never compromise bone bridge thickness below 12 mm.
• Match Composite Thickness to Native: Calibrate the combined thickness of the resected patella and prosthetic button to match or be 1 mm less than native thickness. Avoid anterior overstuffing.
• Preserve Superior Genicular Vessels: If lateral retinacular release is necessary, perform an inside-out release, maintaining a 1.5 cm soft-tissue bridge above the superior pole of the patella to preserve the superior lateral genicular blood supply and prevent patellar avascular necrosis.

Pitfalls

• Internal Rotation Malposition: Internal rotation of the trochlear component causes persistent lateral patellar tilt, subluxation, and early wear. Rescue: Re-seat the anterior cutting guide in external rotation or re-cut the anterior chamfer prior to final cementation.
• Proud Distal Flange: A proud distal prosthetic edge catches the patellar button during flexion, causing a mechanical clunk and anterior pain. Rescue: Ream or countersink the distal transition zone so the prosthetic flange transitions flush into the intercondylar notch cartilage.
• Occult Tibiofemoral Osteoarthritis: Failing to recognize early chondral wear in the medial or lateral compartments leads to rapid failure from disease progression. Rescue: Mandatory preoperative 45° Rosenberg views and direct intraoperative compartment inspection; convert immediately to total knee arthroplasty if full-thickness cartilage defects are identified.
• Excessive Patellar Resection (< 12 mm): Resecting > 50% of patellar thickness dramatically elevates the risk of intraoperative or postoperative patellar fracture. Rescue: If bone bridge is thinned, use an inset button or cement augmentation, and place the patient in a protective hinged brace.
• Unrecognized Elevated TT-TG Distance (> 20 mm): In severe extensor mechanism malalignment, PFA alone will fail due to persistent lateral subluxation. Rescue: Perform a concomitant medialization tibial tubercle osteotomy (Fulkerson procedure) or staged extensor realignment.

8. Postoperative Care

Modern perioperative protocols emphasize immediate weight-bearing, multimodal non-opioid analgesia, and accelerated quadriceps reactivation to optimize kinematic outcomes.

Immediate

• Weight-Bearing Status: Weight-bearing as tolerated (WBAT) immediately on postoperative day 0, utilizing crutches or a rolling walker for balance and stability during the initial 1 to 2 weeks.
• Immobilization: An optional knee immobilizer or hinged brace locked in extension is used for ambulation only until voluntary quadriceps control (unassisted straight leg raise without extension lag) is demonstrated, typically achieved within 24 to 48 hours.
• Multimodal Analgesia: Adductor canal peripheral nerve blockade (preserving motor function to the vastus medialis and rectus femoris), periarticular local infiltration analgesia (LIA), scheduled oral acetaminophen, NSAIDs/COX-2 inhibitors, and oral cryotherapy.
• Venous Thromboembolism Chemoprophylaxis: Low-dose aspirin (81 mg BID) for 4 to 6 weeks in standard-risk patients, or direct oral anticoagulants (DOACs) in patients with elevated thromboembolic risk factors according to clinical guidelines.
• Discharge Pathway: Contemporary patellofemoral arthroplasty is routinely performed as a same-day outpatient surgical procedure or overnight observational stay.

Rehabilitation Protocol

9. Outcome

Patellofemoral arthroplasty restores anterior knee function while maintaining normal joint kinematics and proprioception. When performed for isolated disease using modern onlay implants, clinical outcome scores demonstrate dramatic improvements from preoperative baselines.

• Functional Scores: The Kujala Anterior Knee Pain Scale demonstrates substantial improvements, rising from a preoperative mean of 40–48 points to 82–90 points postoperatively at 2- to 5-year follow-up. The Knee Injury and Osteoarthritis Outcome Score (KOOS) and Oxford Knee Score (OKS) show equivalent or superior early pain relief and functional satisfaction compared to total knee arthroplasty cohorts.
• Range of Motion: Mean postoperative active flexion consistently exceeds 125° to 130°. Preserving both cruciate ligaments and the native tibiofemoral joint line yields an average of 6° to 10° greater flexion compared to matched TKA populations.
• Implant Survivorship: Contemporary onlay prostheses demonstrate 5-year all-cause survivorship rates of 94% to 96% and 10-year survivorship rates between 88% and 92% across international arthroplasty registries (including the Australian Orthopaedic Association National Joint Replacement Registry and the National Joint Registry of England and Wales). In contrast, historical inlay designs exhibited inferior 10-year survivorship (75% to 82%) driven by patellar catching and edge-loading.
• Revision Dynamics: The primary mode of late revision is progressive osteoarthritis in the unresurfaced medial or lateral tibiofemoral compartments (accounting for 60% to 70% of revisions). Revision of an onlay PFA to total knee arthroplasty is straightforward; minimal bone resection and preserved cruciate anatomy permit conversion to standard primary TKA implants without augment stems or constrained components in > 90% of cases, achieving clinical outcomes comparable to primary TKA.

10. Complications

Complications following patellofemoral replacement can be categorized into extensor mechanism maltracking, progressive tibiofemoral degeneration, implant loosening, and soft-tissue contracture.

11. 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.

12. References

• Fredborg C, Odgaard A, Sørensen J. Patellofemoral arthroplasty is cheaper and more effective in the short term than total knee arthroplasty for isolated patellofemoral osteoarthritis: cost-effectiveness analysis based on a randomized trial. Bone Joint J. 2020. doi:10.1302/0301-620X.102B4.BJJ-2018-1580.R3 [Level I: Randomized Controlled Trial]
• Peng G, Liu M, Guan Z et al. Patellofemoral arthroplasty versus total knee arthroplasty for isolated patellofemoral osteoarthritis: a systematic review and meta-analysis. J Orthop Surg Res. 2021. doi:10.1186/s13018-021-02414-5 [Level I: Systematic Review / Meta-Analysis]
• Clar C, Budin M, Rasic L et al. Contemporary patellofemoral arthroplasty is associated with lower revision rates compared with the previous decade: A meta-analysis using revisions per 100 component-years. Knee Surg Sports Traumatol Arthrosc. 2026. doi:10.1002/ksa.70381 [Level I: Systematic Review / Meta-Analysis]
• Cimatti P, Rocchi M, Dallari B et al. Systematic review and meta-analysis of implant-specific outcomes in inlay and onlay patellofemoral arthroplasty for isolated patellofemoral osteoarthritis. J Exp Orthop. 2025. doi:10.1002/jeo2.70541 [Level I: Systematic Review / Meta-Analysis]
• Lewis PL, Graves SE, Cuthbert A et al. What Is the Risk of Repeat Revision When Patellofemoral Replacement Is Revised to TKA? An Analysis of 482 Cases From a Large National Arthroplasty Registry. Clin Orthop Relat Res. 2019. doi:10.1097/CORR.0000000000000541 [Level III: National Arthroplasty Registry Analysis]
• Lonner JH. Patellofemoral Arthroplasty. J Am Acad Orthop Surg. 2007. doi:10.5435/00124635-200708000-00006 [Level V: Master Surgical Review & Consensus]

13. Master Class Pro-Tip

Never compromise on trochlear component external rotation. Positioning the femoral component in slight external rotation (3° to 5° relative to the posterior condylar axis, referenced to the surgical transepicondylar axis) and slightly lateralizing the trochlear groove while medializing the patellar button eliminates lateral tilt and optimizes the Q-angle vector dynamically. Furthermore, verify by direct digital palpation that the transition between the distal prosthetic flange and the intercondylar notch cartilage is completely flush or slightly recessed. A proud distal femoral flange of even one millimeter will cause mechanical impingement against the patellar button during terminal flexion, producing recalcitrant anterior knee pain and early component loosening. If persistent lateral patellar tilt is observed after trial reduction, perform an incremental inside-out lateral retinacular release from within the joint, maintaining a 1.5 cm cuff above the superior pole of the patella to preserve the lateral superior genicular artery and protect the patella from avascular necrosis.

Master Class Surgical Pearl: Patellofemoral Component Alignment and Soft-Tissue Balancing