Patellofemoral Arthroplasty

Indication. End-stage, symptomatic isolated patellofemoral osteoarthritis (or post-traumatic / dysplastic PFJ arthritis) with the tibiofemoral compartments demonstrably intact, that has failed conservative care. "Isolated" is a strict diagnosis: the patient must have Outerbridge grade 2 or less in both tibiofemoral compartments confirmed on weight-bearing radiographs, a Rosenberg view, and either MRI or diagnostic arthroscopy before surgery is offered. Absolute indications

• Isolated patellofemoral osteoarthritis with intact tibiofemoral compartments (Outerbridge grade 2 or less 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 an otherwise normal tibiofemoral joint who is too young for TKA
• Trochlear dysplasia with end-stage PFJ OA where an onlay PFA restores a congruent groove more anatomically than TKA The decision — who is a candidate, who is not. Every PFA begins with the same operation, and the outcome is decided before the incision by patient selection:

Consent specifically for the lifetime risk of conversion to TKA (tibiofemoral OA progression is the leading cause of revision, approximately 20–30 percent at 10 years), patellar maltracking or clunk, anterior knee pain from overstuffing, patellar fracture, infection and the usual arthroplasty risks (DVT/PE). Patients should leave understanding that conversion to TKA is likely within their lifetime. Setup. Supine on a radiolucent table with a bump under the ipsilateral buttock to neutralise hip external rotation; a sandbag or foot-rest holds the knee in 90 degrees of flexion for most of the procedure. A tourniquet sits on the proximal thigh but is inflated only if bleeding obscures visualisation. Anaesthesia is spinal or general with an adductor canal block for postoperative analgesia, plus local infiltration analgesia (ropivacaine, ketorolac and epinephrine) into the capsule and periosteum before arthrotomy. An image intensifier is available but not routinely required; a sterile marking pen, ruler and callipers are essential for measuring patellar thickness and confirming rotation.

The goal is to resurface the patellofemoral joint through a medial parapatellar approach, restore a deep congruent trochlear groove with an onlay component, resurface the patella to native thickness plus 1 mm, and confirm central tracking through a full range of motion — while preserving the tibiofemoral compartments and the option of straightforward later conversion to TKA (see the medial parapatellar approach to the knee page). Anatomy that governs the operation. The trochlear groove runs approximately 7 degrees valgus to the femoral shaft and deepens from proximal to distal. In trochlear dysplasia the proximal groove is shallow or convex, so the patella escapes laterally in early flexion; the onlay component restores a deep, congruent groove that engages the patella from 10–15 degrees of flexion onward. The extensor restraints and the landmarks you will reference are: - 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 inserting at the superomedial patella; protect it during the approach.
• Lateral retinaculum — may require release if tight lateral structures contribute to maltracking.
• TT-TG distance — measured on CT; greater than 20 mm indicates a need for tubercle medialisation.
• Whiteside's line — the anteroposterior axis of the femur, perpendicular to the surgical transepicondylar axis; sets trochlear rotation.
• Surgical transepicondylar axis — from the lateral epicondyle prominence to the medial epicondylar sulcus; the gold-standard reference for femoral component rotation in both PFA and TKA. Structures at risk: the infrapatellar branch of the saphenous nerve crosses the medial joint line during medial parapatellar arthrotomy (postoperative numbness or neuroma), and the superior lateral geniculate artery is encountered during lateral retinacular release and should be coagulated to prevent haemarthrosis.

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The problem it solves. When a patient has isolated patellofemoral arthritis plus one tibiofemoral compartment worn — usually the medial — while the other tibiofemoral compartment and both cruciates are pristine, a TKA sacrifices two healthy structures. Bicompartmental arthroplasty resurfaces only the two diseased compartments and keeps the lateral compartment and the cruciates.

Two ways to do it. Modular ("mix-and-match") — a separate PFA and a separate medial UKA implanted together, so each component is sized and positioned independently; the more forgiving option. Monolithic — a single implant spanning both compartments, technically demanding because one implant must fit two joints and leaves little room to adjust.

Evidence and role. Selected series report function approaching TKA with more natural kinematics and cruciate preservation, but monolithic designs had high early revision from fit and overstuffing problems, and the modular approach lacks long-term comparative data. It remains a niche option for younger patients at high-volume centres, not a mainstream alternative to TKA.

The caveat. It roughly doubles the technical difficulty — two articulations to balance — and, like PFA alone, does nothing to stop osteoarthritis progressing in the untouched lateral compartment.

Rehabilitation | Phase | Timing | Weight-bearing & bracing | Therapy & goals | |-------|--------|--------------------------|-----------------| | 1 | 0–2 weeks | Weight-bearing as tolerated with crutches or walker; hinged brace locked in extension for first 48 h, then unlocked | Passive and active-assisted ROM from day 1; goal 90 degrees flexion by 2 weeks; quadriceps sets and straight-leg raises | | 2 | 2–6 weeks | Full weight-bearing without aids once quadriceps control is adequate | Progressive active ROM; goal full extension and 120 degrees flexion by 6 weeks; stationary cycling, swimming, patellar mobilisation | | 3 | 6–12 weeks | Return to work and light recreation | Closed-chain strengthening, proprioceptive training; full activities as tolerated by 3 months | | Long term | 3 months onward | No high-impact sports (running, jumping, pivoting) to prolong implant survival | Annual radiographic surveillance for tibiofemoral progression | Complications

Conversion of PFA to TKA Indications for conversion are progression of tibiofemoral osteoarthritis causing pain and functional limitation, component loosening or malposition causing persistent symptoms, and patellar maltracking not correctable by revision of the PFA components. Technically, the medial parapatellar approach is reopened, the trochlear component is removed with osteotomes and a saw, and the distal femur is recut in measured-resection fashion using the existing intramedullary canal or extramedullary alignment. The patellar component is revised to a primary TKA button. In the majority of cases a primary TKA system without stems or augments is sufficient (stems or augments are required in less than 15 percent of conversions, and only when significant bone loss occurred at the index procedure). Outcomes after conversion are equivalent to primary TKA when it is performed for progression rather than for component malposition.

Epidemiology. Isolated patellofemoral osteoarthritis — disease confined to the patellofemoral joint with preserved tibiofemoral compartments — accounts for a minority of symptomatic knee osteoarthritis. It is seen in three broad groups: trochlear dysplasia (often younger women, bilateral), post-traumatic arthritis after patellar or trochlear injury, and idiopathic disease in older patients. It is more common in women and is frequently bilateral. Patellofemoral arthritis of some degree is far more common than truly isolated disease, which is why rigorous confirmation of intact tibiofemoral compartments is the whole basis of selecting a patient for PFA. Pathoanatomy and why the onlay component works. In a native knee the trochlear groove deepens from proximal to distal and engages the patella from early flexion. In trochlear dysplasia (Dejour types B–D) the proximal groove is shallow or convex, so the patella escapes laterally; abnormal contact stresses then destroy the articular cartilage. PFA is uniquely effective in this group because the onlay trochlear component restores a deep, congruent groove that the dysplastic native trochlea never had — correcting the primary pathology rather than merely replacing a worn surface. This is why patients with dysplasia have the best long-term results. Evolution of the design — inlay to onlay. 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 proximally, allow independent rotation control, and create a smooth proximal transition that reduces patellar clunk. Contemporary onlay series report 10-year survivorship greater than 85 percent when strict patient selection is applied. When performed for truly isolated PFJ OA, PFA gives reliable pain relief and functional improvement comparable to TKA in the first 5–7 years, with the advantages of preserving bone stock and allowing easier later conversion. The dominant failure mode and conversion data. The leading cause of revision after PFA is progression of tibiofemoral osteoarthritis (approximately 20–30 percent at 10 years) — not failure of the patellofemoral components themselves. This is why counselling every patient about the lifetime risk of conversion is mandatory. Reassuringly, conversion of a failed PFA to TKA is technically straightforward through the same medial parapatellar approach: primary TKA components are sufficient in the great majority, stems or augments are needed in less than 15 percent, and outcomes are equivalent to a primary TKA when conversion is for progression rather than component malposition.