Patellofemoral joint replacement: the Bristol perspective

Abstract

With improvements to implant design, a better understanding of patellofemoral biomechanics, and a sound understanding of the correct surgical indications, patellofemoral joint replacement (PFR) has become an attractive surgical option for patients with isolated patellofemoral osteoarthritis (PFOA). However, PFR is a specialist procedure, with a number of essential pre- and intraoperative considerations that surgeons preforming the procedure must be cognisant of. The aim of this paper is to discuss a number of salient points that our institutions have found to be of great importance following over 30 years of experience in assessing and managing patients with PFOA and subsequent PFR.

Introduction

Patellofemoral osteoarthritis (PFOA) has long been overlooked as a subgroup of knee osteoarthritis (OA), but is now recognized as a distinct clinical entity from tibiofemoral OA. The crude prevalence of radiographic PFOA amongst the adult population has been reported at 25%, increasing to 39% in those aged over 30 years with knee pain, with females being twice as likely to develop the disease.

The management of PFOA can be non-operative or operative. Non-operative strategies can be either pharmacological (e.g. topical non-steroidal anti-inflammatory drugs [NSAIDs], oral NSAIDs, intra-articular glucocorticoid injection) or non-pharmacological (e.g. exercise, weight loss, taping/bracing). However, their effects can be short-lived, and there is a paucity of evidence assessing their efficacy in isolated PFOA. Once non-operative treatment has been exhausted, two forms of joint replacement surgery exist: total knee replacement (TKR) and patellofemoral replacement (PFR). In PFR, only the patellofemoral joint (PFJ) is resurfaced, preserving the tibiofemoral joint surfaces and the cruciate ligaments. PFR is relatively bone conserving, has comparable postoperative functional scores, is associated with less blood loss and shorter hospital stays, and is more cost-effective when compared to TKR.

Registry data reports the risk of revision following PFJ to be at least 2.9 times higher than following cemented TKR across all age groups, with 17% and 24% requiring revision by 10 years and 15 years, respectively. The heterogeneity of practice in registry data, coupled with low surgeon volumes and a lower revision threshold when compared to experienced centres, goes some way to explaining these findings. PFR is a specialist procedure, with a number of essential pre- and intraoperative considerations that surgeons preforming the procedure must be cognisant of. The aim of this paper is to discuss a number of salient points that our institution has found to be of great importance following over 30 years of experience in assessing and managing patients with PFOA and subsequent PFR.

Patient selection for patellofemoral joint replacement

The importance of selecting appropriate patients for PFR cannot be overstated. The most common indication for subsequent revision surgery is progression of tibiofemoral disease. Therefore, it follows that proper pre-operative scrutiny to confirm isolated PFOA is paramount. This is done through a thorough and comprehensive patient history, physical examination and targeted investigations.

Given the patella acts as a mechanical pulley to increase quadriceps muscle torque during knee range of movement, it is reported that anomalies in the mechanics or cartilage of the PFJ can have a large impact on the joint stresses triggering PFOA. The forces generated across the PFJ have been measured at three times body-weight during stair climbing, and nearly eight times body-weight during squatting. Biomechanical abnormalities that affect the PFJ in isolation should be sought, and more often than not, patients will have a previous history of anterior knee pains in their teenage years and early 20s (often labelled as ‘growing pains’) or a history of patellofemoral instability, with either frank dislocations or pseudo-instability (subjective feeling of instability). Additional factors known to increase PFJ stresses should be assessed for, including obesity, repetitive squatting/kneeling and heavy lifting, and previous patellar fracture. Patients with isolated PFOA typically present with anterior knee pain that occurs on standing from a seated position, stair climbing or kneeling. The pain is often less severe when walking on the flat, and when the knee is extended.

The physical examination must include the entire lower limb. Factors affecting the tracking or congruency of the patellofemoral articulation disrupt the normal loading of the joint, and have been associated with an increased risk of PFOA. These include abnormal trochlear morphology, patella alta, lateral patellar tilt, and PFJ instability. Assessment of coronal plane alignment and muscle bulk must be undertaken, as inadequate gluteal or core strength can lead to overuse of the quadriceps and PFJ overload. Similarly, PFJ biomechanics can be altered due to axial plane deformities, and while more relevant in younger patients with instability, the Staheli rotational profile should be assessed in all patients. Issues such as persistent femoral anteversion or external tibial torsion may require further investigation with cross-sectional imaging to accurately assess the torsional profile.

Imaging is used to both confirm PFOA and exclude tibiofemoral involvement. A patellar skyline view can be useful to assess joint space narrowing, subchondral sclerosis, patellar tilt and subluxation, though this may be dependent upon the flexion angle at which the radiograph has been taken. Trochlear dysplasia is poorly represented on patellar skyline views for this reason, and is better appreciated using weight-bearing lateral knee radiographs, which also allow assessment of patellar height and the presence of osteophytes. Tibiofemoral disease is excluded through weight-bearing antero-posterior (AP) and Rosenberg views of the knee (45° flexed postero-anterior). Coronal malalignment can be further quantified with long-leg radiographs, with particular care taken to ensure that true AP views are attained: assessment of a true AP knee view is confirmed with correct proximal tibiofibular overlap and a true projection of the tibial spines. If the ‘patella forward’ technique is used, a laterally subluxed patella (as commonly seen in PFOA) will result in internal rotation of the lower limbs and often an ‘apparent valgus’ of the distal femur due to the normal femoral bow. Torsional abnormalities tend to present in younger patients with instability, but if a significant rotational abnormality is suspected, a computed tomography (CT) rotational profile can provide better assessment of axial malalignment. In addition to plain radiographs, magnetic resonance imaging (MRI) is useful to confirm isolated PFOA and exclude other pathology, and it allows precise measurement of PFJ morphology. Studies have demonstrated a strong association between increased femoral sulcus angle, decreased femoral sulcus depth (dysplasia), patella alta and decreased lateral trochlear angle with PFOA in patients under 50 years old.

There are important contraindications to PFR that must be considered prior to listing patients for surgery. As discussed, it is imperative to exclude tibiofemoral involvement, as well as an inflammatory aetiology. Care must be taken with patients with severe coronal plane deformity (>8°valgus or >5° varus) or fixed flexion deformity (>10°), as this can affect patellar tracking and balance, and may even be representative of another underlying pathology. Increased body mass index (BMI) may contribute to tibiofemoral OA and subsequent need for revision; therefore, patients with a BMI >30 should be counselled carefully about this. Active infection, patella baja, complex regional pain syndrome, and tibiofemoral instability are additional contraindications.

Although studies have failed to identify individual factors predictive of a good outcome following PFR, it has been reported that patients with an earlier history of patellar instability or anterior knee pain experience less progression of radiographic tibiofemoral OA. This makes logical sense, as patients with abnormal patellofemoral biomechanics as a result of dysplasia or alta are more likely to progress to arthritis, due to increased contact forces when compared with their ‘normal’ tibiofemoral joint mechanics. Whilst there may be a benefit in PFR for some elderly patients with isolated PFOA, given the relatively smaller surgical insult, patients younger than 40 or older than 60 report worse outcomes following PFR. Therefore, the surgical candidate that might expect to benefit the most from PFR is middle-aged, likely female, with a background of patellofemoral instability, in whom the joint replacement and associated soft-tissue procedures will address the underlying biomechanical and anatomical aetiology of the disease.

Surgical technique using the Avon prosthesis

PFR surgery has been practiced since the 1950s. Implant design has progressed from a vitallium prosthesis screwed into the patella, to first-generation inlay prostheses available from the 1970s (with the implants functioning as a trochlear articular surface replacement), to second-generation on-lay prostheses (with implants replacing the entire anterior compartment of the knee) from the 1990s. The Avon PFJ prosthesis (Stryker, USA) was designed and developed in the Avon Orthopaedic Centre, Bristol, UK, and has been used since September 1996. It has remained the primary implant for PFR at our institution since its conception. Given this long-standing association, and the philosophy of treating patellofemoral disorders in Bristol, our institution has performed a large number of Avon PFRs and has acquired vast experience in factors contributing to both successful and unsuccessful implantation. Furthermore, the Avon is the most commonly used PFR in the UK National Joint Registry, with 6952 procedures recorded. Unlike more recently released implants, it is a non-sided implant; therefore, a comprehensive understanding of patellofemoral mechanics are needed. Whilst essential for correct placement of the Avon, these principles are relevant to the correct usage of all implants. What follows is a description of a number of fundamental steps that must be considered in order to provide the optimum chance of correct implant positioning and long-term success.

Set-up and approach

The patient should be positioned supine on the operating table, with a side-support and foot bolster, with the knee flexed to 90° (the hyper-flexed position required to access the tibia in TKR is not needed). Prior to skin incision, intravenous antibiotics and tranexamic acid should be administered.

The skin incision should extend from approximately 2 cm proximal to the patella to the level of the tibiofemoral joint line. Full-thickness skin flaps are raised. On the medial side, this must be sufficient to define the anatomy of the arthrotomy, but often more extensively on the lateral side, extending into a full extra-articular release. This step is often required later to improve patellar tracking; therefore, it is often performed as part of the approach. A standard medial parapatellar arthrotomy is performed, exposing the distal anterior femoral cortex proximally and the intermeniscal ligament distally. In order to improve visualization, and thus protect the menisci, cruciate ligaments, and tibiofemoral joint, the distal arthrotomy can be completed with the knee in extension, with the scalpel turned away from the joint. The patella is then everted, and a lateral peri-patellar release performed. There is often thickened lateral patellofemoral and patellotibial bands, which should be released from their femoral origin. A final examination of the tibiofemoral joint is performed to ensure that no OA changes are present necessitating conversion to TKR.

Having adequately exposed the PFJ, attention is turned to preparation for the trochlear component, which must be appropriately positioned in all three axes of the knee. The initial anterior cut is the most important step in the operation, as it ensures correct rotation and flexion of the implant.

Sagittal plane positioning

The Avon system utilizes an anterior cutting guide to prepare the anterior distal femoral cut. This references from the anterior femoral shaft, and appropriate positioning is essential. The flange of the jig is placed on the anterior femur, and further flexed to contact the step of the jig before placing the central femoral pin ( Figure 1 ). Given the increased radius of curvature in the distal femur, failure to correctly position the trochlear component in the sagittal plane may lead to increased anterior offset and overstuffing of the anterior compartment, leading to maltracking and post-operative anterior knee pain. An angel wing placed in the anterior cutting slot should demonstrate flexion relative to the distal third curvature of the femur, and this can be checked intraoperatively by viewing the distal femur from the side. The importance of the anterior cut cannot be underestimated, and this technique can be used using all modern on-lay implants. The subtleties of how to achieve this flexed cut will be unique to each system, but a gentle manual flexion force of the rod of the intramedullary aligned jig systems prior to placement of the stabilization pins will often create the desired effect.