chapter 11: Articular Cartilage Lesions in Patellofemoral Pain Patients
ANATOMICAL ASPECTS OF PATELLAR CARTILAGE BREAKDOWN
The clinical expression of the principal forms of patellar cartilage degeneration does not always allow their differentiation. The arthroscope now permits much more accurate evaluation of patellar articular cartilage lesions.
This is a frequently encountered lesion. The extent to which this causes pain remains unclear, although the arthroscopic surgeon frequently sees soft patellar cartilage as an incidental finding in patients with pain elsewhere. It consists of simple softening of articular cartilage, which begins in a very localized area and then extends progressively in all directions. The initial appearance may be that of a small blister. Macroscopically, the surface is intact (Fig. 11.2). It appears that this is the fundamental and initial lesion of patellar articular cartilage degeneration.
Because the surface is intact in this earliest of all cartilage lesions, it may pass unrecognized. It is important to emphasize that articular cartilage always be palpated with a blunt instrument at arthroscopy as there is not always a difference in surface color or appearance to draw attention to abnormality. Softening, which may at times appear fluctuant, may be present in varying degrees of severity, from simple softening to a more advanced form in which a type of "pitting edema" can be observed after digital or blunt instrument pressure.
The loss of elasticity, which this softening represents, decreases the function capacity of cartilage and explains the reaction of adjacent subchondral bone to which the compression forces are transferred abnormally. One can even wonder how these abnormal areas can stand up against the pressures developed in the patellofemoral joint. This alteration of the physical properties of cartilage helps us to understand better the mechanical destruction of cartilage by fissure formation, fragmentation, and eventual ulceration, as well as certain symptoms (catching, giving way). The discovery of this localized area of softening also explains the computerized tomographic (CT) or magnetic resonance imaging (MRI) finding of localized loss of cartilage, whereas the standard axial radiograph shows a normal joint space. Surrounding normal cartilage maintains the overall normal appearance of the joint, whereas a tomographic study with progressive knee flexion will pick up specific narrowing on a localized surface of the patella.
The techniques of MRI and CT make possible the early diagnosis of this minimal lesion, but are not usually necessary in clinical practice unless the clinician needs to evaluate patellar tracking preoperatively. Extension of the initial malacic lesion, which leads to overall joint narrowing, will be evident eventually on the standard axial radiograph. The finding of this lesion in a radiographically "normal" joint gives significance to the slightly narrowed "joint line," which tends to be underestimated. This incipient lesion will pass completely unobserved in most patients until the time of arthroscopy.
Inside Chapter 11: