CHAPTER SYNOPSIS
Complications involving the patellofemoral joint are the most common reasons for revision surgery following total knee arthroplasty. Patellar maltracking is largely preventable, however, with accurate surgical technique including precise femoral and tibial component positioning. In those patients who present with patellar subluxation postoperatively, the exact etiology of maltracking must be determined in order to choose the appropriate treatment strategy. Imaging studies including computed tomography scanning should be performed, followed by revision arthroplasty for malrotated components. In those with satisfactory position of the components, soft tissue realignment procedures may be considered. The purpose of this chapter is to identify, prevent, and outline techniques in surgical management of patellar maltracking following total knee arthroplasty.
IMPORTANT POINTS
- 1
Component rotation is critical to optimize patellar tracking.
- 2
CT is the most accurate way to assess rotation.
- 3
Intraoperative tracking can be assessed using the “no thumbs” or “single suture” technique.
- 4
Lateral release is indicated for patellar maltracking with well-positioned components. The superior lateral geniculate artery should be preserved during lateral release.
- 5
If subluxation persists following lateral release, a proximal soft tissue realignment procedure may be considered.
- 6
Revision total knee arthroplasty is indicated for poorly positioned components.
CLINICAL/SURGICAL PEARLS
- 1
The femoral component should be positioned in slight external rotation, parallel to the transepicondylar axis.
- 2
Component rotation should be confirmed with secondary landmarks including Whiteside’s Line and the posterior condylar axis.
- 3
The tibial component should be externally rotated, aligned with the junction of middle and medial one third of the tibial tubercle.
- 4
Overall patellar thickness should be maintained, to avoid overstuffing of the patellofemoral joint.
- 5
In revision total knee arthroplasty, anatomic landmarks are distorted, necessitating the use of the transepicondylar axis to determine component rotation.
- 6
A posterolateral augment may be necessary in revision TKA to ensure that appropriate external rotation is achieved.
CLINICAL/SURGICAL PITFALLS
- 1
In the presence of a hypoplastic lateral femoral condyle, use of the posterior condylar axis for referencing during primary total knee arthroplasty will lead to internal rotation of the component.
- 2
Anteriorization of the femoral component during revision arthroplasty will lead to overstuffing of the patellofemoral joint and subsequent maltracking.
- 3
Creating a geometric fit between the tibial tray and the proximal tibial cut will tend to internally rotate the component.
- 4
Due to tibial bone loss during revision surgery, the tendency to use a thicker polyethylene insert will lead to joint line elevation and subsequent patella baja.
- 5
Isolated lateral release in the presence of component malrotation will likely fail; revision arthroplasty is required before soft tissue procedures for extensor mechanism balancing.
HISTORY/INTRODUCTION/SCOPE OF THE PROBLEM
Despite modern advances in technique and design, patellofemoral complications remain the most common cause of failure following total knee arthroplasty (TKA). Often overlooked in the success of this otherwise durable procedure, patellar complications are currently the most common reason for reoperation after TKA. Among these complications of the extensor mechanism, patellar maltracking is the most likely reason for failure after TKA, with a reported incidence of up to 29%. Furthermore, the presence of maltracking is often the source of other patellar complications, such as catastrophic wear, loosening, and fracture. The purpose of this chapter is to identify, prevent, and outline techniques in the surgical management of patellar maltracking following TKA.
CAUSES OF PATELLAR INSTABILITY
Patellar tracking is affected by multiple factors, each of which must be carefully considered during primary or revision TKA. Factors contributing to maltracking include component malrotation, limb misalignment, prosthetic design, patellar preparation, and soft tissue imbalance. Any element that increases the overall Q angle of the limb will increase the laterally directed vector on the patella, thereby increasing the tendency for lateral subluxation. Among the causes of instability, malrotation of the femoral or tibial components is the most common.
Malpositioned Components
Malposition of the femoral, tibial, or patellar components is the predominant cause of patellofemoral instability following TKA. In fact, the combined amount of internal rotation of the femoral and tibial components has been shown to correlate directly with the severity of patellofemoral instability. Furthermore, improper selection of component size is another important factor in patellar maltracking following TKA. An excessively large femoral or patellar component can lead to overstuffing of the patellofemoral joint, resulting in maltracking. An accurately performed arthroplasty with precise femoral and tibial component positioning and sizing is crucial to ensure central patellar tracking and a successful knee replacement.
Femoral Component
Internal rotation and medial translation of the femoral component move the trochlea more medial relative to the extensor mechanism, thus increasing Q angle and predisposing the patella to lateral subluxation. An excessively large femoral component will translate the patellofemoral axis anteriorly, overstuffing the compartment and again increasing the tendency for lateral displacement.
Tibial Component
An internally rotated tibial component increases Q angle by effectively moving tibial tubercle laterally. The resultant increase in the lateral quadriceps vector leads to maltracking of the patella. The size of the tibial component may also play a role in the tendency for malposition. Since the cut surface of the tibial plateau is asymmetric and internally rotated, a larger component creates a geometric fit in internal rotation, with subsequent maltracking ( Fig. 26-1 ).
Patellar Component
Lateral placement of the patellar button on the cut surface of the patella will fail to reproduce the normal median eminence. If the anatomy of the patella has not been restored, subluxation can result. Excessive resection of the lateral patellar facet may also contribute to the maltracking.
Extensor Mechanism Imbalance and Limb Malalignment
A tight lateral retinaculum, combined with laxity or weakness of the medial soft tissue structures, creates a laterally directed force on the patella. This soft tissue imbalance of the extensor mechanism leads to maltracking. Medial laxity may be due to rupture of the medial capsular repair (due to inadequate repair or suture failure), hematoma, overly aggressive physical therapy, or trauma in the early postoperative period.
Extensor mechanism imbalance is especially frequent in patients with limb malalignment from preexisting severe valgus knee deformities. Such individuals, with overall limb alignment of greater than 10 degrees of valgus, have an increased lateral vector of the quadriceps muscle tending to subluxate the patella ( Fig. 26-2 ). Furthermore, patients with valgus knee deformities have significant bone loss or hypoplasia of the lateral femoral condyle. This often leads to internal rotation of the femoral component if a posterior condylar referencing guide is used.
Component Design
There are several elements of femoral component design that play a significant role in patellar tracking. Unlike earlier knee designs with relatively flat trochlear surfaces, modern knee designs have incorporated various features that make them “patella friendly.” A laterally oriented trochlea, for example, engages the patella early in flexion, decreasing the tendency for lateral subluxation. Similarly, a femoral component with a high lateral flange and deep constrained groove will tend to centralize the patella.
Unfortunately, like most constrained components, the decreased tendency for subluxation comes at the expense of increased patellofemoral contact forces. Although instability may be reduced, the abnormal wear can lead to an increased incidence of patellar fracture and component failure. Additionally, it is critical to understand that in the presence of other factors that are contributing to subluxation, increasing the constraint of the trochlea alone will not centralize the patella. In fact, the increased constraint may only serve to further tighten the lateral retinaculum, thereby increasing the tendency for maltracking.
CLINICAL DIAGNOSIS OF INSTABILITY
History
A complete history is the initial step in evaluating a patient with any complication following TKA. Patients with patellar maltracking may present with a variety of complaints. Some may complain of anterior knee pain with a subjective sense of subluxation or dislocation. Others may have no overt symptoms of instability, complaining only of peripatellar pain and limited knee flexion. These patients may have noticed a limitation in their ability to achieve full flexion postoperatively due to subtle patellar subluxation. Maltracking, therefore, should always be considered in the presence of a stiff, painful knee. Patients with a medial retinacular disruption may report a traumatic episode of popping with new onset of pain, or frank patellar dislocation.
Physical Examination
A thorough physical examination of the entire lower extremity is necessary to identify the exact cause of patellar maltracking. The overall alignment of the limb should be assessed, and the presence of excessive valgus alignment and/or tibial torsion should be noted. Tracking of the patella should be observed during knee flexion, and patellar mobility should be assessed. Limitation of patellar mobility may suggest lateral retinacular tightness as a factor contributing to maltracking. Both active and passive motion of the knee should be observed, as this can occasionally help to differentiate between static and dynamic imbalance of the extensor mechanism. Observation of the patella during range of motion may reveal a “J sign,” which is an indication of lateral retinacular tightness. While the patella is engaged within the trochlear groove during flexion, the lateral pull of the retinaculum causes the patella to deviate laterally in full extension.
Imaging
Initial imaging should include standard standing anteroposterior, lateral, and Merchant view radiographs. The anteroposterior view provides valuable information on the degree of valgus angulation of the femoral component. The size of the femoral component is best assessed on the lateral view. The Merchant view provides information on patellar tilt and the angle of patellar resection. A laterally subluxated patella seen on this view will confirm the diagnosis of instability. Recently, some have promoted the use of a weight-bearing axial view of the knee as a way to evaluate patellar tracking under more physiologic conditions.
Patellofemoral instability resulting from malrotation of the tibial or femoral components (or both) is often difficult to diagnose based on physical examination and standard knee radiographs. In addition, the preoperative assessment of rotational alignment is critical in determining whether component revision will be required. Based on a protocol described by Berger et al., computed tomography (CT) has become the study of choice to quantify rotational malalignment. This assessment of alignment is based on two validated and reproducible landmarks, the transepicondylar axis and the tibial tubercle. A cadaveric study by Jazrawi et al. confirmed the accuracy of CT protocols in determining rotational alignment of the components following TKA ( Fig. 26-3 ).
INDICATIONS/CONTRAINDICATIONS
Patellar instability should be managed based on its exact etiology; the precise cause of instability must be identified and corrected for the solution to be successful. Simply performing a lateral release for all cases of maltracking is inappropriate, as this does not always address the cause of the patellar maltracking.
Nonsurgical Management
Nonsurgical measures for management of maltracking include strengthening of the vastus medialis obliquus muscle and bracing to control patellofemoral subluxation. Mild subluxation in patients with a weak vastus medialis may respond to intensive physical therapy, however, nonsurgical treatments are generally unsuccessful. Patellofemoral instability is usually secondary to a component malrotation or other factors that necessitate revision arthroplasty.
Algorithm for Management of Maltracking
See Figure 26-4 .
SURGICAL TECHNIQUE
Prevention of Maltracking
Although patellofemoral maltracking problems can sometimes be successfully treated, the majority of these complications can be avoided altogether with proper surgical technique at the time of primary TKA. Several basic technical guidelines should be considered in order to achieve patellar stability.
Component Position
Femoral Component
Intraoperative evaluation of femoral rotation is critical to ensure proper patellar tracking and may be achieved using one of several anatomic landmarks. These landmarks include the transepicondylar axis, the posterior femoral condylar axis, and the anteroposterior axis. While most surgeons will choose one landmark by which to initially judge rotation (generally the transepicondylar axis), secondary landmarks should be used to confirm the proper position of the femoral component.
- •
The transepicondylar axis connects the lateral epicondylar prominence and the medial sulcus of the medial epicondyle; the femoral component should be parallel or slightly externally rotated to this line.
- •
The anteroposterior axis of the femur, also known as Whiteside’s line, is a line through the center of the trochlear groove and the top of the intercondylar notch. This is a useful secondary landmark to ensure proper femoral component placement. The anterior and posterior femoral cuts should be made perpendicular to Whiteside’s line.
- •
The posterior condylar axis is a line that crosses both posterior femoral condyles, averaging 3 to 5 degrees of internal rotation relative to the transepicondylar axis. The femoral component, therefore, should be slightly externally rotated on the femur relative to the axis of the posterior femoral condyles to accommodate central patellar tracking. Most modern posterior-referencing guides will externally rotate the cut in relation to the posterior condyles, making the component parallel to the transepicondylar axis.
Although the posterior condylar surface is a routinely used system for referencing, it is one of the least reliable landmarks and may lead to a surgical pitfall with resultant patellofemoral maltracking. In a patient with a deficient lateral femoral condyle (due to hypoplasia, longstanding valgus deformity, or prior trauma), using the posterior condylar axis to determine femoral rotation will lead to inadvertent internal rotation of the component. In these cases, the transepicondylar axis is a more accurate guide for component rotation ( Fig. 26-5 ).
In addition to proper rotation, the femoral component should be lateralized in the coronal plane to reduce tension in the lateral retinaculum and to allow for central patellar tracking ( Fig. 26-6 ).
Related posts:
Soft Tissue Balancing during Total Knee Arthroplasty
Intraoperative Complications during Total Knee Arthroplasty: How to Get Out of Trouble
Addressing Ligament Deficiency in Revision Total Knee Arthroplasty
The Infected Primary Total Knee Arthroplasty
Adjunctive Fixation in Total Knee Arthroplasty Revision: Stems and Sleeves
Surgical Exposure in Total Knee Arthroplasty
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