While degenerative changes of the patellofemoral joint exist in at least half of patients with knee osteoarthritis, isolated patellofemoral osteoarthritis is not rare, and a recent meta-analysis revealed radiographic evidence of patellofemoral osteoarthritis in 17% of healthy individuals and 20% of patients presenting knee pain.¹,² Risk factors include trauma, female gender, advanced age, increased body mass index (BMI), quadriceps and abductor weakness, as well as local anatomic factors such as trochlear dysplasia and valgus alignment of the lower extremity.³,⁴ These patients may have significant impact on their quality of life, although it should be most profound with regard to activities that load the anterior knee and less impactful when patients are standing upright and walking on level surfaces.⁵,⁶ Various nonsurgical and surgical options have been described for the management of patellofemoral arthritis.⁷ Patellofemoral arthroplasty (PFA) is an effective treatment for isolated arthritis of the patellofemoral joint, especially after failure of reasonable nonoperative treatments. Total knee arthroplasty (TKA) was regarded as the standard and reproducible treatment for these patients in the past; however, it significantly alters knee kinematics and would sacrifice the otherwise-healthy tibiofemoral compartments.⁸ Additionally, given that 50% of PFA candidates are 50 years old or younger,⁹ TKA may be undesirable for many patients with isolated patellofemoral arthritis, particularly in light of recent data showing improved function after PFA compared to TKA performed for isolated patellofemoral arthritis.¹⁰,¹¹,¹² Thus, PFA has gained greater interest as understanding of selection criteria has evolved and designs have improved.⁹,¹³,¹⁴

Perhaps the most critical factor determining the success of PFA remains appropriate patient selection. Indications for PFA include patients with isolated patellofemoral arthritis (or Outerbridge grade IV lateral patellar facet and/or lateral trochlear chondromalacia) due to primary osteoarthritis, posttraumatic arthritis, or secondary to trochlear dysplasia, patellar maltracking/subluxation, and/or patellar instability. Pain should be shown to be inadequately responsive to nonoperative treatment measures, which may include physical therapy, weight loss, nonsteroidal anti-inflammatory medication, activity modification, injections, or bracing.

On the other hand, this procedure should not be offered to individuals with pain on the medial or lateral aspects of the knee or evidence of arthritis or grade III-IV chondromalacia in the tibiofemoral compartments, as well as patients with inflammatory arthritis, chondrocalcinosis, fixed flexion contracture more than 10°, or uncorrected patellofemoral or tibiofemoral malalignment, as these suggest more diffuse pathology which cannot be adequately managed by isolated PFA.¹⁵ While mild to moderate patellar maltracking or patellar tilt is easily addressed at the time of PFA with lateral retinacular release or recession and appropriate positioning of the trochlear and patellar components, severe patellofemoral malalignment or rotational deformity, noted on clinical examination and confirmed with imaging, is a relative contraindication if not correctable prior to, or simultaneous with, PFA. Patella baja, patellar tendon scarring, and quadriceps weakness are other reported relative contraindications to PFA.⁴ Of note, PFA may be used in conjunction with a unicompartmental knee arthroplasty (so-called bicompartmental knee arthroplasty) in patients with medial or lateral tibiofemoral degenerative disease.¹⁶,¹⁷ Additionally, focal femoral condylar cartilaginous lesions noted on preoperative magnetic resonance imaging (MRI) or at the time of PFA can be effectively addressed with combined PFA and osteochondral grafting.¹⁸

Intuitively, due to the increased patellofemoral stresses associated with increased weight, obese patients are thought to be at increased risk of failure after PFA, but more of an issue, is that obese patients are more likely to have subtle or overt tibiofemoral disease which can compromise the results of PFA. Indeed, this has been confirmed by previous studies, demonstrating that obese patients (BMI >30 kg/m²) are at higher risk for revision for a variety of reasons.¹⁹,²⁰ However, to date, there is no accepted BMI cutoff for PFA. Similarly, there is currently no consensus regarding optimal age for patients undergoing PFA, though authors have generally advocated for a younger patient population (30-60 years old).²¹,²² In one series, 50% of patients undergoing PFA were age 50 years or younger. Nonetheless, excellent outcomes are achievable even in octogenarians with isolated patellofemoral arthritis.⁹ We would not typically recommend PFA for patients in their 20s.

Additional contraindications include active infection, complex regional pain syndrome, disproportionate pain, and narcotic dependence to manage patellofemoral pain. Patients who require opioid medications for patellofemoral osteoarthritis are generally considered poor candidates for PFA, and all attempts should be made to wean them from these medications prior to pursuing surgery. Last, previous studies have shown that coexisting psychological distress or psychiatric disease may be associated with poorer outcomes and/or poorer satisfaction postoperatively.⁹ Accordingly, it is important for the practitioner to determine the mental status of patients prior to proceeding with PFA and set appropriate and realistic expectations for patients. Indications and contraindications are further summarized in Table 21-1.

Preoperative evaluation includes a thorough history and physical examination. The history usually reveals anterior (retropatellar or peripatellar) knee pain which is exacerbated during activities that particularly stress the patellofemoral joint such as descending stairs, kneeling, squatting, sitting for a prolonged period of time, or going from a sitting to a standing position. There should be little if any pain when walking on level ground. There may be a history of patellar trauma, dislocation, or patellar instability. Nonoperative treatment measures and prior procedures should be documented.

Physical examination is focused on the assessment of the patellofemoral joint as well as more global evaluation of the knee and lower extremity to exclude other pathology. Active patellar tracking is assessed with the limb dangling over the edge of the examination table. Typically, patellofemoral crepitus is felt and/or heard. Patellar maltracking may be observed with lateral deviation of the patella as the knee approaches full extension (J sign), indicating muscular imbalance or rotational deformity. For patients who have high Q angles, a tibial tubercle realignment procedure (anteromedialization) may be considered before, or at the same time as, PFA. Assessment for hypermobility, patella alta, and baja is also useful. Provocative testing should include elicitation of tenderness with palpation around the patella, apprehension with attempted lateral subluxation, pain and crepitus with patellar compression, and recreation of patellofemoral crepitus and retropatellar knee pain with range of motion and squatting.

The examiner should also look for other causes of anterior knee pain such as patellar tendonitis, pes anserinus bursitis, synovitis, complex regional pain syndrome, referred pain from the hip or the lumbar spine…etc. Tenderness to palpation at the joint line medially or laterally should also be evaluated and would contraindicate PFA even in the absence of tibiofemoral arthritis on radiographic imaging.

Preoperative imaging includes four views of the knees (weight-bearing anteroposterior, weight-bearing mid-flexion posteroanterior, lateral, and sunrise views). These should identify the presence of patellofemoral degenerative changes and the absence of tibiofemoral arthritis, though small osteophytes and mild squaring of the femoral condyles may be acceptable in the context of normal tibiofemoral joint spaces and lack of clinical symptoms. Patellar tilt, subluxation, and patellar congruence are evaluated on the sunrise view. Patellar height should also be assessed on the lateral X-ray. If significant lower limb angular deformity is suspected, full-length standing plain films should be obtained.

MRI is routinely obtained when evaluating a patient being considered for PFA. While it is naturally used to confirm the findings of patellofemoral joint degeneration (chondral thinning, bony edema), equally important is its role in excluding the presence of substantial tibiofemoral compartment pathology such as meniscal injury, chondromalacia/arthritis, or subchondral edema. The presence of more substantial tibiofemoral chondral disease or edema would exclude isolated PFA, although consideration may be given to bicompartmental knee arthroplasty, combined PFA and chondral grafting, or TKA in these circumstances. Previous arthroscopy photographs or video, if available, may be especially valuable in documenting the extent of patellofemoral joint disease as well as the absence of disease elsewhere.

Implant design in PFA has witnessed significant progress over the years. Initial prostheses in the 1950s consisted of patellar resurfacing with vitallium implants, leaving the native trochlea untouched.²³ Later implants in the 1970s addressed the femoral trochlea and constituted first-generation PFA prostheses. These implants have an “inlay” design, whereby the trochlear implant only replaces the articular cartilage and sits flush with the surrounding cartilage.¹⁴,²¹,²⁴ These implants follow the alignment of the native trochlea—typically internally rotated 10° relative to the AP axis of the femur—which is the primary reason for high rates of secondary surgery and revision of inlay-style trochlear designs, mostly related to abnormal patellofemoral tracking and instability.²⁵,²⁶,²⁷

Recognizing the etiology of failures with inlay-style trochlear designs, second-generation prostheses—considered an “onlay” trochlear design—were developed to be positioned flush with the anterior femoral cortical surface and perpendicular to the anteroposterior axis (Whiteside axis) of the femur.¹⁴,²¹,²⁴ The ability to rotationally position the trochlear implant independently of the native anterior trochlear anatomy has substantially improved patellofemoral tracking, reduced the need for secondary surgeries or revisions, and optimized functional outcomes and durability.¹³,¹⁴ Several additional design features have contributed to the improvements in patellar tracking, including a less constrained trochlear groove, an anatomic radius of curvature, greater proximal extension, improved congruence, and asymmetric designs.¹³