Patellofemoral arthroplasty has a long record of use in the treatment of isolated patellofemoral arthritis, with outcomes influenced by patient selection, surgical technique, and trochlear implant design. The trochlear components have evolved from inlay-style to onlay-style designs, which have reduced the incidence of patellar instability. Minimizing the risk of patellar instability with onlay-design patellofemoral arthroplasties has enhanced mid-term and long-term results and leaves progressive tibiofemoral arthritis as the primary failure mechanism beyond 10 to 15 years.
Key points
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Patellofemoral arthroplasty (PFA) has a long record of use in the treatment of isolated patellofemoral arthritis, with outcomes influenced by patient selection, surgical technique, and trochlear implant design.
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The trochlear components have evolved from inlay-style to onlay-style designs, which have reduced the incidence of patellar instability.
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Inlay-design trochlear prostheses are inset within the native trochlea, flush with the surrounding articular cartilage. The component rotation is therefore influenced by the native trochlear inclination, which tends to be internally rotated relative to the anteroposterior and transepicondylar axes of the femur, accounting for the high incidence of patellar instability with inlay-design components.
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Onlay-design trochlear components are implanted perpendicular to the anteroposterior axis of the femur, resecting the anterior trochlear surface flush with the anterior femoral cortex and positioning the implant irrespective of the native trochlear inclination, which is the number one reason for the significant improvement in patellar tracking with onlay-style trochlear implants.
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Minimizing the risk of patellar instability with onlay-design PFAs has enhanced mid-term and long-term results and leaves progressive tibiofemoral arthritis as the primary failure mechanism beyond 10 to 15 years.
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Revision PFA to an onlay-design is reasonable to consider in the situation of a failed inlay-style trochlear prosthesis, if no tibiofemoral arthritis is present. Otherwise, revision to total knee arthroplasty can yield predictable results.
Introduction
Epidemiologic studies indicate that isolated patellofemoral arthritis affects nearly 10% of the population over 40 years of age. In one study, women were more than twice as likely as men to have isolated anterior compartment degeneration (24% vs 11%), likely related to subtle dysplasia and malalignment. As the population ages and the burden of arthritis increases, more patients will likely seek treatment for this condition in the upcoming years. In addition, as younger patients in their 30s through 50s continue to present with isolated patellofemoral arthritis, conservative operative treatments like patellofemoral arthroplasty (PFA) will remain important alternatives to total knee arthroplasty (TKA) when nonoperative interventions are ineffective.
Most patients with patellofemoral arthritis can be treated symptomatically and with nonoperative modalities (including anti-inflammatory medications, physical therapy, weight reduction, bracing, and injections). However, a small percentage of patients may require surgical intervention if these treatments fail. Surgical options include nonarthroplasty procedures (arthroscopic debridement, tibial tubercle unloading procedures, cartilage restoration, and patellectomy) and partial (patellofemoral) or TKA. Historically, nonarthroplasty surgical treatment has provided mixed and inconsistent results, with success rates of 60% to 70%, especially in patients with advanced arthritis. Although TKA provides reproducible results in patients with isolated patellofemoral arthritis, it may be undesirable for those interested in a more conservative, kinematic-preserving approach, particularly in younger patients. Due to these limitations, PFA continues to emerge as a more mainstream option. This review focuses on the historical and contemporary results of PFA as influenced by advances in prosthetic (specifically trochlear component) design.
Indications for PFA
As with any surgical procedure, a prerequisite for good outcomes with PFA is proper patient selection. Therefore, results of any series of PFA should be interpreted in the context of appropriate indications. The ideal candidate for PFA has isolated, noninflammatory anterior compartment arthritis resulting in pain and functional limitations that are persistent despite reasonable attempts at nonoperative treatments. Patients should have only retropatellar and/or peri-patellar pain that is exacerbated by stairs, sitting with the knee flexed, and standing from a seated position. Symptoms should be reproducible during physical examination with squatting and patellar inhibition testing. An abnormal Q-angle or J-sign indicates significant maltracking and/or dysplasia, particularly with a previous history of patellar dislocations. The presence of these findings may necessitate concomitant realignment surgery with PFA. However, with newer prosthesis designs, moderate maltracking can be corrected with proper orientation of the prosthesis and occasionally a lateral release. Often, patients with patellofemoral arthritis will have significant quadriceps weakness, which should be corrected with preoperative physical therapy to prevent prolonged postoperative pain and functional limitations.
Radiographs should be consistent with isolated patellofemoral arthritis, indicated by joint space narrowing and osteophytes on the lateral and Merchant views ( Fig. 1 ). Narrowing within the medial or lateral compartments on weight-bearing views may disqualify that patient from a PFA. The authors also prefer obtaining a preoperative magnetic resonance imaging scan to further evaluate the tibiofemoral compartments for evidence of chondral damage or reactive edema, to guide treatment between PFA and bicompartmental or total knee arthroplasty. Previous arthroscopy photographs are especially valuable in documenting the extent of anterior compartment cartilage loss and the presence or absence of degeneration elsewhere in the knee.
Introduction
Epidemiologic studies indicate that isolated patellofemoral arthritis affects nearly 10% of the population over 40 years of age. In one study, women were more than twice as likely as men to have isolated anterior compartment degeneration (24% vs 11%), likely related to subtle dysplasia and malalignment. As the population ages and the burden of arthritis increases, more patients will likely seek treatment for this condition in the upcoming years. In addition, as younger patients in their 30s through 50s continue to present with isolated patellofemoral arthritis, conservative operative treatments like patellofemoral arthroplasty (PFA) will remain important alternatives to total knee arthroplasty (TKA) when nonoperative interventions are ineffective.
Most patients with patellofemoral arthritis can be treated symptomatically and with nonoperative modalities (including anti-inflammatory medications, physical therapy, weight reduction, bracing, and injections). However, a small percentage of patients may require surgical intervention if these treatments fail. Surgical options include nonarthroplasty procedures (arthroscopic debridement, tibial tubercle unloading procedures, cartilage restoration, and patellectomy) and partial (patellofemoral) or TKA. Historically, nonarthroplasty surgical treatment has provided mixed and inconsistent results, with success rates of 60% to 70%, especially in patients with advanced arthritis. Although TKA provides reproducible results in patients with isolated patellofemoral arthritis, it may be undesirable for those interested in a more conservative, kinematic-preserving approach, particularly in younger patients. Due to these limitations, PFA continues to emerge as a more mainstream option. This review focuses on the historical and contemporary results of PFA as influenced by advances in prosthetic (specifically trochlear component) design.
Indications for PFA
As with any surgical procedure, a prerequisite for good outcomes with PFA is proper patient selection. Therefore, results of any series of PFA should be interpreted in the context of appropriate indications. The ideal candidate for PFA has isolated, noninflammatory anterior compartment arthritis resulting in pain and functional limitations that are persistent despite reasonable attempts at nonoperative treatments. Patients should have only retropatellar and/or peri-patellar pain that is exacerbated by stairs, sitting with the knee flexed, and standing from a seated position. Symptoms should be reproducible during physical examination with squatting and patellar inhibition testing. An abnormal Q-angle or J-sign indicates significant maltracking and/or dysplasia, particularly with a previous history of patellar dislocations. The presence of these findings may necessitate concomitant realignment surgery with PFA. However, with newer prosthesis designs, moderate maltracking can be corrected with proper orientation of the prosthesis and occasionally a lateral release. Often, patients with patellofemoral arthritis will have significant quadriceps weakness, which should be corrected with preoperative physical therapy to prevent prolonged postoperative pain and functional limitations.
Radiographs should be consistent with isolated patellofemoral arthritis, indicated by joint space narrowing and osteophytes on the lateral and Merchant views ( Fig. 1 ). Narrowing within the medial or lateral compartments on weight-bearing views may disqualify that patient from a PFA. The authors also prefer obtaining a preoperative magnetic resonance imaging scan to further evaluate the tibiofemoral compartments for evidence of chondral damage or reactive edema, to guide treatment between PFA and bicompartmental or total knee arthroplasty. Previous arthroscopy photographs are especially valuable in documenting the extent of anterior compartment cartilage loss and the presence or absence of degeneration elsewhere in the knee.
PFA design considerations
PFA was first developed over 30 years ago, although it has remained somewhat controversial until recently because of high failure rates seen with early (and even some contemporary) inlay-style trochlear prosthesis designs ( Fig. 2 A ). With contemporary onlay-style trochlear implants (see Fig. 2 B) that replace the entire anterior trochlear surface and are more optimally positioned, high success rates and good functional outcomes are more easily achievable. Table 1 summarizes key design differences between inlay-style and onlay-style trochlear components.
| Inlay | Onlay | |
|---|---|---|
| Positioning | Inset flush with native trochlea | Replaces entire trochlea, perpendicular to AP axis |
| Rotation | Determined by native trochlea | Set by surgeon, perpendicular to AP axis |
| Width | Narrower | Wider |
| Proximal extension | No further than native trochlear surface | Extends further proximal than native trochlea |
Inlay Style
Initial attempts at PFA used trochlear components inset into the native trochlea, attempting to position the prosthesis flush with the surrounding trochlear articular cartilage ( Fig. 3 ). The resulting design characteristics have proved problematic when coupled with the inherent anatomic variations and inclination of the native trochlea, which make positioning of the component challenging relative to the articular surfaces and biases the component into internal rotation, predisposing to high rates of patellar maltracking, catching, and subluxation.
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The shapes of these components frequently do not match the shape of the trochlea, particularly in the situation of trochlear dysplasia, leading to malpositioning of the prosthesis as it will not sit flush against all surfaces. Several inlay prostheses have large radii of curvature. To avoid impingement of the implant on the anterior cruciate ligament or tibia by a proud inferior aspect of the prosthesis, flexion of these components may be necessary. Flexion of these components results in offset of the proximal aspect of the prosthesis from the anterior femoral cortex, causing catching and subluxation of the patella in the initial 15 to 30° of flexion.
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The rotation of the component is determined by the native trochlear orientation. A recent study by Kamath and colleagues examined trochlear inclination angles in 329 patients with either normal or dysplastic patellofemoral anatomy. Based on magnetic resonance imaging scans, both groups had trochlear inclination angles averaging 11.4° and 9.4° of internal rotation, respectively, relative to the anatomic landmarks (anteroposterior and transepicondylar axes). This finding explains the propensity to internally malrotate inlay-style trochlear components, which predisposes to patellar maltracking and subluxation. Like internally rotated femoral components in TKA, internal rotation of the trochlear component in PFA effectively medializes the trochlear groove, increases the Q-angle, and puts tension on the lateral retinaculum, all of which predispose to patellar maltracking and instability.
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The narrow width and often deep constraining sulcus of some inlay-style trochlear components are more constraining to the patella with little accommodation for patellar tracking, which also increases the potential for patellar maltracking.
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The proximal aspect of the inlay-style trochlear component does not extend proximal to the trochlear articular margin. This proximal aspect often results in the patella not being engaged in the trochlear component when the knee is in full extension, particularly in patients with patella alta. As the knee flexes, the patella transitions onto the trochlear component, which may cause catching and subluxation, particularly if the trochlear component is flexed, offset proximally, and internally rotated.
Onlay Style
Onlay-style trochlear prostheses ( Fig. 4 ) replace the entire anterior trochlear surface, alleviating many of the issues described above when having to accept the constraints of native anatomic aberrations common in this population. This design can be applied to all patients, regardless of anatomic variations, and is therefore more versatile and suitable for general use.
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Most onlay prostheses have anatomic radii of curvature, ensuring the prosthesis sits flush with the anterior femoral cortex proximally and the articular cartilage above the intercondylar notch distally.
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The rotation of the trochlear component is determined by the surgeon intraoperatively based on anatomic landmarks, similar to TKA. The component is positioned perpendicular to the anteroposterior axis (Whiteside’s line) and parallel to the transepicondylar axis, facilitating patellar tracking and eliminating the effect of native trochlear rotation seen with inlay prostheses ( Fig. 5 ).
