The Journal of Bone and Joint Surgery

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Bilateral Acute Knee Pain in a Woman with Systemic Lupus Erythematosus¹
(continued)

Fig. 1

Atraumatic patellar tendon rupture (bilateral). Figure 1 shows superior displacement of the patella (patella alta).

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Discussion

Patellar tendon rupture is the third most common cause of disruption of the extensor mechanism of the knee, after patellar fracture and quadriceps tendon rupture. It is estimated that a force of 17.5 times body weight is required to cause rupture in healthy patients. The injury most commonly occurs in patients younger than forty years of age who overload the extensor mechanism during athletic activity. However, steroid injection, rheumatologic disease, renal failure, infectious disease, and metabolic disorders are associated with an increased risk of patellar tendon rupture.

Bilateral patellar tendon rupture is a rare injury. Of the forty-eight cases reported in the English-language literature, sixteen occurred in the absence of trauma or athletic exertion. Most of the patients had a history of systemic disease or knee surgery.

Detection of patellar tendon rupture may be difficult, so frequently there is a delay in diagnosis. In a series of thirty-six patellar tendon ruptures in thirty-three patients, Siwek and Rao reported that ten ruptures (28%) were misdiagnosed on initial examination and that seven (19%) were neglected (that is, they were repaired more than two weeks after injury).

Patients with patellar tendon rupture present with pain, swelling, and minimal ability to extend the knee. Patients with bilateral injury have symmetrically abnormal findings on physical examination, limiting the value of comparison with the contralateral limb. Physicians must distinguish true extensor lag from limitation of motion secondary to pain. In the absence of fracture, the differential diagnosis includes meniscal injury, quadriceps tendon rupture, and patellar tendon rupture. Disruption of the patellar tendon may be distinguished from that of the quadriceps tendon by palpation of the tendon defect and by the behavior and position of the patella on contraction of the quadriceps. The atypical presentation of some patients with low-energy or atraumatic bilateral injury mandates a thorough history and physical examination to make the proper diagnosis (Table I).

TABLE I Diagnosis of Low-Energy Patellar Tendon Rupture

History

Pain out of proportion to injury

Inability to stand or walk

Giving-way of knee joint

Physical examination

Inability to extend knee

Palpable defect in patellar tendon

Effusion

Radiographs

Patella alta

In patellar tendon rupture, the patella is superiorly displaced and is classically seen as patella alta on lateral radiographs (Fig. 1). The position of the patella may be evaluated on the basis of the ratio of the greatest diagonal length of the patella to the length of the patellar tendon on lateral radiographs (the Insall-Salvati ratio). This measurement is relatively independent of knee flexion, and a ratio of less than 0.80 indicates patella alta (Fig. 2). The diagnosis may be confirmed by magnetic resonance imaging or ultrasound examination.

Fig. 2

Fig. 2. The Insall-Salvati ratio in a normal knee (A) and in one with patella alta (B). LP = length of the patella, and LT = length of the patellar tendon.

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Prompt diagnosis is important because neglected injuries lead to proximal retraction of the patella with scarring, complicated repair, and diminished long-term function. Delayed repair often requires extensive release of scar tissue and use of tendon allograft or harvesting of other tissues for use as autograft. The rehabilitation time is increased due to atrophy that develops during the period of neglect.

Reference

1. Rose PS, Frassica FJ. Atraumatic bilateral patellar tendon rupture: a case report and review of the literature. J Bone Joint Surg Am. 2001;83:1382-6.