chapter 9:Patellar Subluxation
When pain occurs, it is frequently difficult to localize. Retinacular pain (12, 13) may occur, either in the medial or lateral retinaculum. Some patients also demonstrate patellar tendon or distal patellar apophyseal tenderness, probably because of alteration of normal stresses in this area. Occasionally, the clinician will note tenderness in the distal quadriceps, particularly at the vastus lateralis obliquus or vastus medialis obliquus insertion. Care should be taken to palpate all soft tissue components around the patella, including the medial infrapatellar ligament and all components of the lateral retinaculum (13). Apprehension is common on displacing the patella in a lateral direction. Hypermobility of the patella, which reflects excessive ligamentous laxity, may be pronounced. Patella alta is notable in some patients. Medial or lateral parapatellar tenderness is often present. Effusion exists only after an acute episode or when secondary cartilage damage has complicated the disorder. Muscle atrophy is very common, but generally not as marked as with recurrent dislocation. Minor crepitus may be felt during flexion and extension of the knee under the weight of the leg.
Hughston (8) has described a lateral orientation of the patella with the knee flexed to 90 degrees. This is not common in our experience and is more likely associated with tilt‑compression. Occasionally, particularly when the patient is flexing and extending the knee under load (squatting), subluxation can be observed. The patella momentarily perches on the lateral trochlear facet and then suddenly relocates with an audible snap. It is rare to detect this because it usually occurs under functional circumstances that the patient cannot consciously reproduce. However, the patient should be made aware of what to look for so that he/she may more accurately interpret the episodes when they occur and thus facilitate the correct diagnosis.
An abnormal patellar course is a characteristic sign of patellar instability. The importance of observing the course of the patella from the fully extended knee with the quadriceps set to full flexion and back again has been underlined for many patellar abnormalities. It is particularly important to observe the entrance and exit into the trochlea at between 10 and 20 degrees of flexion. The abnormalities here are many, but some that we have encountered are as follows: (1) An abrupt lateral movement at the termination of extension; (2) a trajectory described by Ficat (14) as "bayonet," which is characterized by an abrupt lateral translation just before full extension and then further extension in a straight line. The entire course is one of two vertical lines connected by an abrupt lateral translation; (3) an atypical semicircular route as if the patella were pivoting around the lateral trochlear facet; and (4) a trajectory in which there is a short medial displacement, just before the final slight external movement associated with full extension.
This list is in no way exhaustive and may even vary with a given case, dependent upon what muscles are brought into play and what movements are carried out with regard to rotation or varus/valgus stress during extension. Perhaps the diversity of observations explains some of the confusion that exists in attempting to classify subluxations and dislocations of the patella. It must be remembered, of course, that, at the time of examination, the patellar course may be entirely normal. Once a clinical impression is formed, however, one should confirm the alignment pattern radiographically.
Many patients with simple patellar subluxation have little or no evidence of articular disease (Fig. 9.6). Is it not uncommon to find at arthroscopy that the patellar articular surface is perfectly normal despite considerable apprehension related to instability. However, some patients with patellar subluxation, particularly if there is associated tilt, will eventually develop dysplastic changes (Fig. 9.7), articular cartilage softening, and erosion. Recurrent subluxation accompanied by tilt will lead to lateral facet arthrosis in many patients. Once dislocation has occurred, articular damage is particularly common at the medial patellar facet or the lateral trochlea.
Inside Chapter 9: