Quadriceps tendon rupture as the initial presentation of undiagnosed alkaptonuria (ochronosis): A rare case report

INTRODUCTION

Quadriceps tendon rupture is an uncommon injury, most often associated with systemic comorbidities, chronic tendon degeneration, or significant trauma. In younger and middle-aged individuals, rupture following low-energy mechanisms is unusual and should prompt evaluation for underlying systemic or metabolic disorders.^[1]

Alkaptonuria is a rare autosomal recessive metabolic condition caused by a deficiency of homogentisic acid (HGA) oxidase, resulting in progressive accumulation of HGA within connective tissues.^[2] This leads to ochronosis, characterized by pigmentation, early degeneration of cartilage, ligaments, and tendons, and subsequent mechanical weakening.^[3] Musculoskeletal manifestations typically present in adulthood and most commonly involve the spine and large joints.^[4]

Although Achilles tendon rupture has been more frequently reported in patients with ochronosis, involvement of the quadriceps tendon is exceptionally rare.^[5,6] We present a case of spontaneous quadriceps tendon rupture in a patient with previously undiagnosed alkaptonuria, where the diagnosis was established based on characteristic intra-operative findings and confirmed by post-operative systemic evaluation.

CASE REPORT

A 49-year-old male manual laborer with no comorbidities, intermittent mild back pain, and no relevant family history presented with right knee pain and swelling for 10 days following a low-energy fall from standing height. There was no history of significant trauma, corticosteroid use, systemic illness, or prior tendon pathology.

On examination, there was a palpable suprapatellar gap, swelling of the anterior thigh, and loss of active knee extension. Knee radiographs demonstrated patella baja without bony avulsion, multiple loose bodies, and moderate osteoarthritic changes [Figure 1a]. Magnetic resonance imaging (MRI) confirmed a complete quadriceps tendon rupture at its patellar insertion, with proximal retraction of approximately 21 mm, a transverse defect of 28–30 mm, and multiple intra-articular loose bodies, including one within the tendon at the tear site [Figure 1b].

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Figure 1:

(a) Anteroposterior, lateral, and skyline radiographs of the right knee joint showing patella baja and two large loose bodies. (b) Sagittal fat-suppressed proton density magnetic resonance imaging of the right knee (lateral-to-medial sequence) demonstrating a complete full-thickness quadriceps tendon tear at the patellar insertion with capsular rupture, extension of joint fluid into the subcutaneous plane, and multiple small to medium loose bodies within the suprapatellar bursa, Hoffa’s fat pad, lateral gastrocnemius insertion, and popliteal bursa. A large loose body is also noted within the tendon defect.

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Surgical repair was performed under spinal anesthesia. Diagnostic arthroscopy revealed normal intra-articular structures, and loose bodies were removed. Through a midline incision, the quadriceps tendon was exposed and appeared friable with characteristic blackish-brown discoloration involving both tendon and synovium [Figure 2a]. These findings suggested ochronotic involvement. Debridement was carried out, and primary end-to-bone repair was achieved using two No. 5 FiberWire sutures through transosseous tunnels through the patella [Figures 2b and c]. Biopsy confirmed ochronosis, showing chronic inflammatory changes, multinucleated giant cells, and ochronotic pigment granules within fibrocartilage.

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Figure 2:

(a) Intra-operative image showing quadriceps tendon defect with friable margins and characteristic blackish discoloration of the tendon and synovium. (b) Primary end-to-bone quadriceps tendon repair was performed using transosseous patellar tunnels. (c) Debrided quadriceps tendon specimen demonstrating ochronotic black pigmentation.

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Post-operative recovery was uneventful. Further evaluation was prompted by the intraoperative findings. Lumbar spine radiographs demonstrated narrowed and calcified intervertebral discs [Figure 3a]. A urine sample darkened within 4 h on exposure to air [Figure 3b], consistent with alkaptonuria. Laboratory studies revealed no additional metabolic abnormalities.

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Figure 3:

(a) Anteroposterior and lateral radiographs of the lumbar spine showing degenerative changes with calcified intervertebral discs. (b) Urine sample demonstrating black discoloration after 4 h of exposure to air, consistent with alkaptonuria.

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The patient was immobilized in extension for 2 weeks and kept non-weight-bearing for 4 weeks. Gradual mobilization began at 2 weeks, and by 6 weeks, he had regained full knee extension and returned to daily activities.

DISCUSSION

Alkaptonuria is a rare metabolic disorder, with an incidence of 1 in 250,000–1,000,000 live births.^[2] Progressive deposition of HGA in cartilage, ligaments, and tendons leads to structural weakening and early degeneration.^[5] Ochronotic arthropathy typically presents in the third or fourth decade with back pain, stiffness, and large joint involvement,^[3] while tendon rupture, though much less common, may be the first manifestation.^[5,6]

Extensor mechanism ruptures of the lower limb are exceedingly rare and usually unilateral. Established risk factors include advancing age, diabetes mellitus, chronic kidney disease, hyperparathyroidism, gout, obesity, hypercholesterolemia, and systemic connective tissue disorders such as rheumatoid arthritis or lupus. In younger or middle-aged patients without these comorbidities, spontaneous rupture should raise suspicion of metabolic causes, including alkaptonuria.^[1,7]

The underlying pathology involves chronic HGA accumulation and polymerization into ochronotic pigment, which binds to collagen, causing brittleness and loss of elasticity.^[8,9] This predisposes tendons to degeneration and rupture. While Achilles tendon rupture is most frequently described in ochronosis, quadriceps tendon rupture is exceptionally rare. Mwafi et al.^[5] described extensive ochronotic degeneration of the Achilles tendon, highlighting the vulnerability of tendons in this condition. Chua and Chang^[6] reported bilateral ruptures as the initial presentation of alkaptonuria. In our case, the diagnosis was unsuspected until intraoperative observation of black tendon pigmentation, a pathognomonic feature.

Diagnosis of alkaptonuria is often delayed, as many patients remain asymptomatic until midlife. Classical clinical clues include darkening of urine on standing and bluish-black pigmentation of the sclera or cartilage.^[2] Radiographs typically show intervertebral disc calcification and degenerative changes in large joints. For many, intraoperative recognition of ochronotic pigment is the first diagnostic clue. In our case, lumbar intervertebral disc calcification and urine discoloration supported the intra-operative suspicion and confirmed the systemic nature of the disease. Other reported manifestations include involvement of the hip, knee, and shoulder joints, as well as cardiovascular and renal systems, underscoring the multisystem nature of alkaptonuria.^[2,10]

Surgical repair of ochronotic tendon rupture can be technically demanding due to friable tissue. Both transosseous sutures and suture anchors have been used successfully.^[6] Despite the underlying degeneration, short-term surgical outcomes are usually favorable, though long-term prognosis is limited by progressive systemic disease, especially spinal and large joint arthropathy.^[4]

This case highlights the importance of recognizing unusual intraoperative findings. Orthopedic surgeons, particularly those involved in sports medicine and arthroscopy, may be the first to encounter undiagnosed alkaptonuria. Early recognition allows appropriate surgical management while facilitating timely systemic evaluation, patient counseling, and long-term disease surveillance.

CONCLUSION

Quadriceps tendon rupture can be the first manifestation of previously undiagnosed alkaptonuria. Intraoperative recognition of ochronotic pigmentation is a valuable diagnostic clue. Awareness of this rare entity can aid early diagnosis, appropriate management, and counseling regarding the systemic nature of the disease.