CHAPTER 17 Approach to Chondral Damage in the Patellofemoral Joint

Chondral defects of the patellofemoral joint are the most common type of articular cartilage defect found in the knee.¹ They also comprise a significant portion of most isolated grade IV articular cartilage defects in the knee joint. These lesions may be found incidentally but also may be a common source of pain and discomfort in the knee.² Trochlear articular cartilage defects can be symptomatic. The approach to this joint can be complex and may require a combination of arthroscopic and open surgical techniques, up to and including arthroplasty, which is beyond the scope of this chapter.³^,⁴ I shall primarily discuss arthroscopic techniques for chondral defects.

ANATOMY AND PATHOANATOMY

The patellofemoral joint of the knee is a complex articulation, with multiple facets on the patella and a complex, saddle-shaped trochlear groove.⁵^–⁷ The articular cartilage on the patella itself is the thickest articular cartilage in the body, measuring up to 7 mm thick on the medial facet. Chondral defects on the patella may be have several causes, including degeneration, direct trauma, malalignment, and patellar instability.⁸^–¹⁰ The mechanics of the patellofemoral joint are complex and are determined by a number of factors, including neuromuscular coordination, geometry of the trochlea, overall alignment of the extensor mechanism, including the quadriceps tendon, trochlear groove, and tibial tubercle, and soft tissue restraints such as the medial patellofemoral ligament.³^,¹¹^–¹³ These restraints help guide patellofemoral motion and often determine whether articular cartilage damage occurs and is symptomatic. For example, excessive patellar malalignment with a laterally tilted and subluxated patella can contribute to excessive articular cartilage wear on the lateral facet of the patella.¹⁴^,¹⁵ Direct trauma to the patella, such as from a dashboard-type injury, in which a flexed knee contacts the dashboard, may result in a central articular cartilage damage to the patella or trochlea.¹⁶^–¹⁹ In addition, many central articular cartilage defects are noted in young athletic individuals. These often have a linear fissure in the center of the trochlea. This reflects the significant forces placed on the patellofemoral joint during activities of daily living and other forms of physical exertion. For example, the joint reaction forces on the patellofemoral joint during stair climbing are up to two to four times body weight, and jumping can create forces up to six to eight times body weight.²⁰^–²⁵ These loads can result in failure of the underlying articular cartilage and subchondral bone.

PATIENT EVALUATION

History and Physical Examination

Patients with symptomatic articular cartilage lesions of the patellofemoral joint generally complain of anterior knee pain, particularly when ascending or descending stairs or after sitting.⁴ Occasionally, patients may complain of crepitus or a symptomatic click as the knee goes into flexion or extension. Elements of the history to obtain from the patient also include known or suspected rheumatologic disease, history of trauma to the knee from a direct blow, history of patellar instability, activity level of the patient, recent changes in activity level, timing of the injury, and whether the pain is acute or chronic. In patients who have already sought previous treatment, it is important to identify the nature of treatment, including physical therapy, bracing, and taping and any previous surgery. Complaints of recurrent or consistent effusions should also be identified. Knee pain diagrams often provide relevant clinical information.²⁶

Physical examination should combine static and dynamic evaluation. The presence or absence of any effusion should be noted and the knee should be carefully palpated and evaluated for ligamentous and patellar instability. At 30 degrees of knee flexion, the patella will typically translate laterally approxi mately 9 mm and have a relatively firm end point because of the check rein of the medial patellofemoral ligament.²⁷ The patella should also translate medially 7 mm and the lateral patellar retinaculum should allow the patella to be tilted to a neutral position, where the anterior surface of the patella is parallel to the ground while the patient is supine. Quadriceps development, particularly that of the vastus medialis obliquus muscle, should be assessed.⁴ Extensor alignment including femoral version, Q angle, and tibial rotation can provide insights into patellar tracking.

A specific evaluation of the articular cartilage of the patellofemoral joint should be performed. The knee should go through an active range of flexion and extension and the presence of any significant crepitus should be assessed. I prefer to perform this examination with the patient lying supine, with the hip flexed to 90 degrees and the knee actively flexed and extended against gravity. In my clinical practice, this has been found to identify and elicit patellofemoral or articular cartilage complaints more precisely than while the patient is sitting. This also allows an assessment of hamstring tightness, which may contribute to patellofemoral symptoms. Another specific test to assess for chondral defects in the patellofemoral joint is the Clark patellar compression test. Two fingers of the examiner are held at the top of the patella to stabilize it in the trochlea with the knee extended, and the patient is asked to activate the quadriceps mechanism. This tightens the quadriceps and forces the patella into the patellofemoral joint; it is positive if pain is elicited. In addition, the medial and lateral facets of the patella can sometimes be directly palpated for tenderness. However, unlike many parts of the femoral tibial articulation, the central trochlea usually cannot be directly assessed by palpation.

The Q angle with the knee extended is of limited benefit. The Q angle with knee flexed to 30 degrees is a more accurate representation of the actual location of the patella and the trochlea groove compared with the location of the tibial tubercle.

Dynamic examination of the knee should be performed as well. Abnormalities of gait should be identified.²⁸ Evaluation of knee stability and identification of pain or crepitus while doing a step-up or step-down maneuver may help elicit specific complaints. A single leg squat test may also elicit pain and will allow evaluation of relative knee valgus or internal rotation, which can increase patellofemoral forces.

Diagnostic Imaging

Radiography

An anteroposterior (AP) radiograph can provide information about the existence of a bipartite patella. The relative location of the patella to the trochlear groove and tibial tubercle can be determined as a secondary bony density on the proximal tibia.

A true lateral view with overlapping femoral condyles, with less than 5% displacement, can provide valuable information regarding the relative patellar height. This can be assessed using various ratios, including the Insall-Salvati index and Blackburne-Peel method (Fig. 17-1)—the articular surface length compared with the distance from its inferior margin to the tibial plateau. This technique has been demonstrated to be the most reliable. A true lateral view can also yield information about the degree of trochlear dysplasia. The trochlear groove can be identified on the lateral x-rays, on which a dense white line of the trochlea should not intersect vvthe bone of the lateral femoral condyle (Fig. 17-2). A sunrise or Merchant view at 45 degrees of flexion can give indicate patellar tracking within the trochlea and the presence of lateral patellar tilt or subluxation. A number of measurements have been described; however, I agree with Merchant that “(f)or most patients, it is quicker and just as good to eyeball the film rather than spend time drawing lines and calculating measurements.”²⁹

FIGURE 17-1 Methods of measuring patellar height. Normal for all ratios 1:1(± 0.2)—Blackburne-Peel (most reliable), A:Y; Caton, A:X; Insall-Salvati, B:Z.

(Adapted from Merchant AC. Patellofemoral imaging. Clin Orthop Relat Res. 2001;(389):15-21.)

FIGURE 17-2 Lateral radiograph demonstrating trochlear dysplasia. The arrow shows the trochlear groove. Note how it starts after the curve of the femoral condyle.

Magnetic Resonance Imaging

Magnetic resonance imaging (MRI) is currently the best modality to image articular cartilage defects.³⁰^,³¹ Appropriate articular cartilage sequences and thin cuts often reveal articular cartilage damage, such as chondromalacia, chondral flaps, or delamination. In addition, it can also identify areas of underlying subchondral bony edema. MRI is also useful to assess abnormalities of soft tissue that may be contributing to underlying chondral damage or patellofemoral pain, such as synovitis, lesions of the medial patellofemoral ligament, tendinopathy, and articular cartilage loose bodies. MRI can also be used to perform measurements of tibial tubercle to trochlear groove distance. This can indicate conditions that could lead to patellar instability and subsequent articular cartilage damage.