Patellofemoral Arthroplasty

Surgical Indications

Absolute Indications

• Isolated patellofemoral osteoarthritis with intact tibiofemoral compartments (Outerbridge grade less than or equal to 2 on arthroscopy or MRI)
• Post-traumatic patellofemoral arthritis after patellar fracture or trochlear injury with preserved tibiofemoral cartilage
• Severe trochlear dysplasia (Dejour type B, C or D) with recurrent instability and end-stage cartilage loss
• Failed extensor mechanism realignment procedures with isolated PFJ degeneration

Relative Indications

• Patient age greater than 50 years with low demand and isolated PFJ disease who wishes to avoid TKA
• Post-traumatic PFJ arthritis in a patient with otherwise normal tibiofemoral joint who is too young for TKA
• Trochlear dysplasia with end-stage PFJ OA where onlay PFA restores a congruent groove more anatomically than TKA

Contraindications

Absolute:

• Any tibiofemoral chondral loss Outerbridge grade 3 or 4 on weight-bearing radiographs, Rosenberg view, or arthroscopy
• Inflammatory arthropathy (rheumatoid, psoriatic, ankylosing spondylitis) or prior septic arthritis
• Uncorrected patellar maltracking (TT-TG greater than 20 mm without planned tubercle osteotomy)
• Fixed flexion deformity greater than 10 degrees or valgus/varus greater than 10 degrees

Relative:

• Age less than 50 years or BMI greater than 35 (accelerated wear and higher revision risk)
• Previous high tibial osteotomy or distal femoral osteotomy altering joint line
• Patellar bone stock insufficient for resurfacing (residual thickness less than 10 mm after resection)

Evidence for Patellofemoral Arthroplasty

Evolution from Inlay to Onlay Designs

Older inlay trochlear components recessed into the trochlear groove had high rates of catching at the proximal transition zone and difficulty controlling rotation. Modern onlay designs sit proud of the native bone, allow independent rotation control, and create a smooth proximal transition that reduces patellar clunk. Contemporary series using onlay components report 10-year survivorship greater than 85 percent when strict patient selection is applied.

Outcomes versus Isolated Patellofemoral Disease

When performed for truly isolated PFJ OA, PFA provides reliable pain relief and functional improvement comparable to TKA in the first 5-7 years, with the advantage of preserving bone stock and allowing easier conversion. However, the leading cause of revision remains progression of tibiofemoral osteoarthritis (approximately 20-30 percent at 10 years). Patients with trochlear dysplasia have the best long-term outcomes because the onlay component corrects the shallow groove that caused their original pathology.

Conversion to Total Knee Arthroplasty

Conversion of a failed PFA to TKA is technically straightforward when the original approach was medial parapatellar. The trochlear component is removed, the distal femur recut in measured-resection fashion, and a primary TKA system is used in the majority of cases. Stemmed components or augments are required in less than 15 percent of conversions. Outcomes after conversion are equivalent to primary TKA when performed for progression rather than component malposition.

Relevant Surgical Anatomy

Patellofemoral Joint Geometry

The trochlear groove is oriented approximately 7 degrees valgus relative to the femoral shaft and deepens from proximal to distal. In trochlear dysplasia the proximal groove is shallow or convex, allowing the patella to escape laterally in early flexion. The onlay trochlear component restores a deep, congruent groove that engages the patella from 10-15 degrees of flexion onward.

Extensor Mechanism Restraints

• Medial patellofemoral ligament (MPFL): primary restraint to lateral translation in the first 30 degrees of flexion; often attenuated in chronic instability.
• Vastus medialis obliquus (VMO): dynamic medial stabiliser; inserts at the superomedial patella and must be protected during approach.
• Lateral retinaculum: may require release if tight lateral structures contribute to maltracking.
• Tibial tubercle-trochlear groove (TT-TG) distance: measured on CT; greater than 20 mm indicates need for tubercle medialisation.

Neurovascular Structures at Risk

The infrapatellar branch of the saphenous nerve crosses the medial joint line and is at risk during medial parapatellar arthrotomy — may cause postoperative numbness or neuroma. The superior lateral geniculate artery is encountered during lateral retinacular release and should be coagulated to prevent haemarthrosis.

Medial Parapatellar Approach

Incision and Arthrotomy

A midline or slightly medial longitudinal incision from 5 cm above the patella to the tibial tubercle is used. The medial parapatellar arthrotomy begins 2 cm medial to the patellar border, curves around the patella leaving a 5 mm cuff of quadriceps tendon and capsule medially, and extends distally along the medial border of the patellar tendon to the tibial tubercle. This approach provides excellent exposure of the entire trochlea and patella while preserving the lateral retinaculum for later assessment of tracking.

Key Landmarks

• Proximal trochlear extent: identified by palpating the anterior femoral cortex proximal to the trochlear groove.
• Whiteside's line: the anteroposterior axis of the femur, perpendicular to the surgical transepicondylar axis — used to set trochlear rotation.
• Surgical transepicondylar axis: line connecting the lateral epicondyle prominence to the medial epicondylar sulcus — the gold-standard reference for femoral component rotation in both PFA and TKA.

Positioning and Preparation

Patient position: Supine on a radiolucent table with a bump under the ipsilateral buttock to neutralise hip external rotation. A sandbag or footrest holds the knee in 90 degrees of flexion for the majority of the procedure. A tourniquet is applied to the proximal thigh but is not inflated unless bleeding obscures visualisation.

Anaesthesia: Spinal or general anaesthesia with adductor canal block for postoperative analgesia. Local infiltration analgesia (LIA) with ropivacaine, ketorolac and epinephrine is infiltrated into the capsule and periosteum before arthrotomy.

Setup: Image intensifier available but not routinely required. A sterile marking pen, ruler, and callipers are essential for measuring patellar thickness and confirming rotation.

Step-by-Step Operative Technique

Step 1: Exposure and Assessment

Perform the medial parapatellar arthrotomy. Evert the patella laterally and inspect the entire trochlear groove, patellar articular surface, and both tibiofemoral compartments under direct vision. Confirm that tibiofemoral cartilage is Outerbridge grade less than or equal to 2; if not, abort PFA and proceed to TKA. Measure native patellar thickness with callipers in the midline before any resection.

Step 2: Patellar Resurfacing Preparation

Resect the patellar articular surface with a patellar clamp or oscillating saw aiming for a residual bone thickness of 12-14 mm. The resection plane should be parallel to the anterior patellar cortex. Drill peg holes or prepare the surface according to the implant system. Protect the resected patella in saline-soaked gauze while preparing the trochlea.

Step 3: Trochlear Component Sizing and Rotation

Select the trochlear component size that covers the entire native trochlea from proximal to distal without overhang. Set rotation using an extramedullary guide aligned perpendicular to Whiteside's line or parallel to the surgical transepicondylar axis. The component should be lateralised 2-3 mm relative to the native sulcus in valgus knees. Mark the position with electrocautery before any bone preparation.

Step 4: Trochlear Bone Preparation

Prepare the trochlear bed according to the implant system (milling, burring, or saw cuts). The proximal transition zone must be flush or slightly proud to prevent patellar catching as it enters the groove in early flexion. Remove all osteophytes and ensure the component seats fully without rocking.

Step 5: Trial Reduction and Tracking Assessment

Insert trial components. Reduce the patella and assess tracking through a full range of motion. The patella should engage the trochlear groove by 15 degrees of flexion and remain centred without lateral tilt or subluxation throughout. If tracking is poor, perform lateral retinacular release or revise the trochlear rotation before proceeding. The "no thumb" test (patella tracks without manual pressure) must be satisfied.

Step 6: Final Component Implantation

Cement the trochlear and patellar components using standard PMMA technique. Remove excess cement carefully, especially from the proximal trochlear transition zone. Reassess tracking after cement polymerisation with the final implants.

Step 7: Wound Closure and Assessment

Close the arthrotomy with interrupted absorbable sutures. If a lateral release was performed, close the lateral retinaculum loosely or leave open if tracking remains perfect. Close subcutaneous tissue and skin. Apply a compressive dressing and a hinged knee brace locked in extension for the first 24-48 hours.

Implant Selection

Trochlear Component Design

Modern onlay trochlear components are preferred. They sit proud of the native bone proximally, allow independent rotational positioning, and provide a smooth transition that reduces patellar clunk. Inlay designs are largely obsolete because of higher rates of catching and difficulty controlling rotation.

Patellar Component

A three-pegged or single central peg patellar button is used. Thickness must restore native patellar thickness plus 1 mm. Metal-backed patellar components are avoided because of higher loosening rates.

Fixation

Both components are cemented with PMMA. Cementless trochlear fixation has been described but is not widely adopted because of concerns about early micromotion in the trochlear bed.

Complications and Management

Postoperative Rehabilitation Protocol

Phase 1 (Day 0-14)

• Weight-bearing as tolerated with crutches or walker for 1-2 weeks
• Hinged brace locked in extension for first 48 hours, then unlocked for ROM exercises
• Passive and active-assisted range of motion beginning day 1 — goal 90 degrees flexion by 2 weeks
• Quadriceps setting and straight-leg raises with brace on
• Ice and elevation for swelling control

Phase 2 (Week 2-6)

• Full weight-bearing without assistive devices once quadriceps control is adequate
• Progressive active ROM — goal full extension and 120 degrees flexion by 6 weeks
• Stationary cycling, swimming, and low-impact strengthening
• Patellar mobilisation and scar massage

Phase 3 (Week 6-12)

• Return to work and light recreational activities
• Closed-chain strengthening, proprioceptive training
• Full activities as tolerated by 3 months

Long-term Restrictions

• No high-impact sports (running, jumping, pivoting) recommended to prolong implant survival
• Annual radiographic surveillance for tibiofemoral progression

Conversion of PFA to Total Knee Arthroplasty

Indications for Conversion

• Progression of tibiofemoral osteoarthritis causing pain and functional limitation
• Component loosening or malposition causing persistent symptoms
• Patellar maltracking not correctable by revision of PFA components

Technical Considerations

The medial parapatellar approach is reopened. The trochlear component is removed with osteotomes and a saw. The distal femur is recut in measured-resection fashion using the existing intramedullary canal or extramedullary alignment. Patellar component is revised to a primary TKA button. In the majority of cases a primary TKA system without stems or augments is sufficient. Outcomes are equivalent to primary TKA when conversion is performed for progression rather than technical failure of the index PFA.