ISOLATED ARTICULAR CARTILAGE LESIONS OF THE FEMORAL TROCHLEA
FRCS (Tr & Orth) Research
Institute of Musculo-Skeletal Research
*Currently : Cunningham Specialist Centre 1 / 10 Pring Street Ipswich Queensland 4305 Australia
A retrospective review of 1317 consecutive arthroscopies was undertaken to confirm the presence of an isolated femoral trochlea chondral lesion. 79 patients were identified with significant trochlear lesions. 11 of these had isolated trochlea chondral injuries without concomitant retropatella, condylar or other internal derangement of the knee. We describe the aetiology of this isolated lesion and discuss the potential mechanisms of injury.
KEY WORDS Femoral Trochlea, Isolated Articular Lesion
Articular cartilage injuries can be associated with other internal derangements of the knee or, less frequently can be seen in isolation. The innate capacity for such defects is poor and progression to osteoarthritis is well recognised. Whilst newer methods of treatment have realised some success with injury of the condylar surfaces, lesions of the patello-femoral joint remain recalcitrant to treatment (1).
The patello-femoral joint is a complex articulation within the knee joint. This leads to a spectrum of disorders from instability to articular injury. This paper identifies an isolated lesion of the femoral trochlea articular cartilage and discusses possible mechanisms for its occurrence.
The purpose of this study was to identify the incidence and aetiological factors of isolated femoral trochlea articular injuries. A retrospective review of 1317 consecutive arthroscopies performed by the senior author, over a 4-year period, was undertaken. Arthroscopic findings were compared with the patient’s pre-operative clinical profile and diagnosis. Articular injury was graded according to the method of Outerbridge (I-IV) (2).
Arthroscopic debridement of all loose chondral fragments was performed. Post-operatively patients underwent a patello-femoral rehabilitation program under physiotherapy supervision.
From the consecutive series of 1317 patients, 79 patients were identified with clinically significant trochlea lesions. 68 patients had ‘mirror’ changes on the retro-patella surface. Only 11 patients had isolated femoral trochlea articular cartilage lesions (Outerbridge II/III) with no other articular defect of the retro-patellar or condylar surfaces, or other internal derangement (Figure 1).
Of these 11 patients, 9 were male and 2 female with an average age of 35 years (range 18 - 46 years). With one exception, all were recreational athletes involved in running, racquet or contact sports. The last patient was a competitive skier. Only 6 patients recalled a specific injury, which was either a direct blow to the anterior patella surface, or a twisting injury about a semi-flexed knee. Those recalling a specific injury were of a younger age (< 35 years).
All patients presented with pain and tenderness localised to either medial or lateral joint line. Thus meniscal injuries were the commonest pre-operative diagnosis (9 patients). Only 2 patients had suspected patello-femoral pathology pre-operatively. An effusion was present in 8 patients, indicating a synovial response to articular debris. 5 patients described ‘catching’ and 2 described true locking. Only 1 patient had clinically obvious patello-femoral malalignment but no previous history of subluxation or dislocation. There were no radiographic abnormalities. The isolated femoral trochlea injury was only confirmed at arthroscopy.
At the 6-week post-operative review all pre-operative symptoms had settled. There were no residual effusions and no localised pain or tenderness. All symptoms of ‘catching’ and locking had resolved. However, we await the long-term outcome in these patients.
Our understanding of the pathology of the patello-femoral joint remains incomplete. Although the senior author originally described an isolated femoral trochlea lesion in 1978 (3), no further reports have been published. It is hoped that this paper will accurately profile this injury and thus contribute to our growing knowledge of this complex articulation.
Anatomically, the trochlea involves the intercondylar region of the femur and has an extension onto the medial condyle for articulation with the patella (4). Functionally, it serves to centralise the extensor mechanism between 20 and 90 degrees of flexion. The extensor mechanism acts predominantly as a decelerator, through eccentric contraction in this range. Patello-femoral contact pressures have already been calculated for a number of activities. In athletic pursuits, requiring sudden deceleration on the flexed knee, eccentric contraction of the extensor mechanism can produce pressures exceeding 6 to 7 times body weight (5, 6). Chondral injury under such supraphysiological loading is possible.
To date, most discussion of patello-femoral pathology has focused on the patella (7). Trochlea lesions have been described, though invariably in association with advanced chondromalacia of the patella. Insall (8) considered chondromalacia of the femoral trochlea uncommon and followed no particular pattern. Trochlea changes are generally considered to mirror injury on the patella surface. Outerbridge (2) and Jackson (9) agreed that trochlea injury was identified with higher grades of chondromalacia patellae, and this can be seen in their respective classifications of that condition. Zamber et al (10) assessed articular lesions arthroscopically and identified 7 trochlea lesions. All but one was associated with grade III changes of the patella. In these cases, the patella lesions were significantly more severe than that seen in the trochlea. Thus it has been postulated that patellar injury precedes disease on the femoral trochlea. Few studies of patello-femoral injury challenge this impression that trochlea injury is a secondary phenomenon. Even recent scintigraphic assessment found no specific pattern of injury in the trochlea (11).
The available evidence suggests that trochlea lesions are traumatic in origin. The mechanisms of articular cartilage injury have been examined in biomechanical and animal models (5, 12, 13, 14). These models demonstrate that both compressive and shear forces can produce this type of lesion, but that shear is more important. Furthermore, shear injury at different speeds and energies can realise a spectrum of reproducible chondral and osteochondral trauma. In the patello-femoral joint, even direct forces to the patella will have a shear component because of the convex morphology of the distal femur.
The structure of hyaline cartilage will fail with acute impact loading above a certain threshold and in response to repetitive sub-maximal loads. The anatomical and biochemical changes in ageing articular cartilage are well documented (15). Significant change is frequently evident by 35 years of age. These structural changes may render aged cartilage susceptible to injury and explain the genesis of these lesions in older athletes.
The unanswered question is why the sudden, or repetitive increases in patello-femoral contact pressures does not equally traumatise the patella. Based on our current understanding we may hypothesise that under certain conditions the patella may be conferred protection by: a) its relative mobility; b) its circumferential soft tissue attachments may dissipate some of the forces imparted; c) the material properties and thickness of patella articular cartilage may enhance its own viscoelastic properties. Furthermore a relative mismatch of material properties in the patella and the trochlea has been demonstrated (16). Patella cartilage is less stiff, with a higher water content and permeability than trochlea cartilage. These changes do not vary with age.
The isolated femoral trochlea chondral lesion is uncommon. Its existence highlights the complexity of patello-femoral articulation and the need for ongoing research. High shear and compressive forces may lead to its evolution. Deceleration of the flexed knee, with co-contraction of the hamstrings and quadriceps groups can generate sufficient contact pressures to produce this injury. Our findings challenge the impression that femoral trochlea injury is always secondary to pre-existing patella pathology.
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