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chapter 10: Patellar Dislocation

Pathoanatomy

Articular injury is common after acute patellar dislocation. The force necessary to dislocate a patella in most patients is substantial, particularly when one considers that the medial retinaculum must be substantially stretched and torn to permit the patella to drop off the lateral trochlea. Thomson (personal communication, 1988) has shown that most patients with acute patellar dislocation will have loose osteochondral fragments in the joint with injury to the medial patellar facet and lateral trochlea. Frequently, an unimpressive radiogram may accompany patellofemoral injury in which a large fragment of articular cartilage has been sheared away from the lateral femoral trochlea or medial patella (Fig. 10.2). Small bone flakes on radiograph sometimes accompany large sections of displaced articular cartilage. The arthroscopic evaluation of patients with acute patellar dislocation has revealed that this is not a benign condition. One must consider that pre‑existing malalignment has been aggravated by substantial articular injury, particularly to the medial patella and lateral trochlea, with medial retinacular disruption. For these reasons, acute patellar dislocation should be regarded not only as an extension of subluxation, but rather as a major traumatic event with implications of significant additional structural damage.

Treatment

It has been customary to reduce the acutely dislocated patella and immobilize the ex­tremity for a period of time to permit healing of the medial retinaculum, with presumed restabilization of the patella. Larsen and Lauridsen (31) have confirmed that conserva­tive management is appropriate for many patients after a first‑time patellar dislocation. Arthroscopic evaluation of such patients, however, by Neal Thomson (personal com­munication, 1988) and others has demonstrated that articular injury may be substan­tial, and arthroscopy, therefore, may be appropriate for many patients who have sus­tained an acute patellar dislocation. Hawkins et al (32) noted that those patients with a predisposing history of underlying malalignment may benefit from immediate arthro­scopic intervention. Even in the face of unremarkable radiograms, articular injury may be substantial, and the best opportunity to restore significant displaced fragments is im­mediately following surgery. If the lateral retinaculum is particularly tight, lateral reti­nacular release at this time may be helpful in re‑establishing a normal or near‑normal tracking pattern. Vainionpaa et al (33) prospectively studied 55 patients treated surgically for acute patellar dislocation and noted a redislocation rate of 9 percent, with most patients returning to the preoperative level of athletic activity.

Although immobilization may be the treatment of choice by many orthopedic surgeons, we believe that an arthroscopy to evaluate these patients and surgery aimed at restoring major displaced fragments of articular cartilage or releasing major deforming forces, such as a tight lateral retinaculum, may improve the long‑term prognosis for such patients. More extensive realignment surgery, however, should be avoided in the acute setting. Cofield and Bryan (34) noted a 42% failure rate after nonsurgical treatment for patellar dislocation. Certainly, the surgeon who undertakes arthroscopy of a patient with acute patellar dislocation must be prepared to correct the significant articular lesions identified.

If the treating physician is confident that there is no major articular fragment displacement, and if the patella can be reduced well into the trochlea and maintained there after aspiration of the hemarthrosis,, a period of 6 weeks' bracing  may restore adequate stability. In general, we will use a Trupull patella support brace (DJO, Vista, CA) Nonoperative treatment may give satisfactory results, particularly when there is no pre‑existing congenital abnormality of the extensor mechanism (32, 35). The clinician should consider arthroscopy selectively. Diagnostic arthroscopy adds little risk and can add substantially to detecting serious damage to a patient's extensor mechanism. Simple replacement of osteochondral fragments and selective lateral retinacular release (particularly in patients with lateral tilt) may help avoid future problems when there is evidence of causative malalignment. Bigos and McBride (36) noted that there was no recurrence of dislocation in 23 knees at a mean follow‑up time of 14.5 months after lateral release for dislocation. Dainer et al (37), however, noted less satisfactory results after lateral release for acute patellar dislocation, but their patients were not selected for pre‑existing malalignment. Major reconstruction, however, should generally be avoided immediately after an acute patellar dislocation. We do not consider lateral release alone to be adequate treatment for patella dislocation.

 

 

 

 

        

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