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Calcific Tendinitis of the Patellar Tendon with MRI Correlations

Background: Calcific tendinitis is a predominantly cell mediated disease that has been well documented in multiple areas of the body, but very rarely in the patellar tendon. This study presents an unusual case of severe patellar calcific tendinitis initially diagnosed as mucoid degeneration and partial patellar tendon tear after a sports injury. Three modalities of diagnostic imaging were unable to identify calcific deposition which was only discoverable intra-operatively. We describe this unusual presentation and optimal imaging sequences for diagnosis. Case Report: A 51-year-old male presented three weeks after injuring his left knee playing tennis. The patient lunged forward with his left foot to hit a ball when he felt a sudden sharp pulling sensation at his anterior knee over the patellar tendon. This pain was most pronounced with weight-bearing activities and did not improve over the next month. Left knee radiographs were unremarkable; MRI and ultrasound images demonstrated diffuse patellar tendon partial thickness interstitial tearing and what appeared to be mucoid degeneration. None of the standard imaging J Orthop Sports Med 2021; 3 (3): 116-122 DOI: 10.26502/josm.511500047 Journal of Orthopaedics and Sports Medicine 117 modalities identified calcification. Surgical exploration of the patellar tendon revealed a large pocket of classic calcific deposition under pressure which was evacuated prior to tendon repair. The patient had rapid resolution of his pain and was full weight bearing at two weeks post-operation. Conclusion: Calcific tendinitis of the patellar tendon is an unusual condition that can present diagnostic difficulty. Commonly used imaging modalities and protocols may be insufficient in diagnosing calcification. If tendon calcification is suspected, consideration may be given to the use of gradientecho sequences as a part of MRI imaging for definitive diagnosis and treatment optimization.


Introduction
Calcific tendinitis is a cell mediated disease, characterized by the metaplastic transformation of tenocytes into chondrocytes, resulting in calcific deposits in tendons [1]. The progression of calcific tendinitis through a pre-calcific phase, calcification phase, stable phase, and resorptive phase has been well documented. Initially, vulnerable sites undergo cellular changes predisposing them to calcification in the pre-calcific phase. This is followed by cellular secretion of chalky deposits in the calcification phase and a subsequent painless stable phase. The final resorptive phase is most painful and may last for weeks while the calcium deposit develops a toothpaste-like appearance [2]. Clinically, patients with calcific tendinitis may present with acute onset or chronic pain and restricted range of motion [3].
Calcific tendinitis is most commonly seen in the rotator cuff tendons of the shoulder; their incidence is as high as 2.7%, with approximately 35% occurring asymptomatically [3]. Calcific tendinitis has also been documented at the wrist, hip, thigh, knee, ankle, and foot [3]. Very few cases of calcific tendinitis of the patellar tendon have been documented. Calcification can be visualized on radiographic, ultrasound, and MR imaging but is typically non-specific. On radiographic imaging, calcific deposits may appear well defined (in the calcification phase) or translucent (in the resorptive phase). On ultrasound examination of the shoulder, these deposits usually appear hyperechoic in the calcification phase or isoechoic in the resorptive phase. Calcifications appear as vague regions of low signal on T1 and T2 MR imaging [4].
Tendon calcification can often be missed on standard imaging, and in very rare cases, may mimic tendon rupture [5].
We present here a rare case of severe patellar calcific tendonitis initially diagnosed as mucoid degeneration and partial tear of the patellar tendon after a sports injury. Three modalities of diagnostic imaging including ultrasound, radiographs, and MRI were unable to correctly diagnose calcific deposition. The diagnosis was made after surgical exploration.

Case Report
A 51-year-old male with a BMI of 32 presented three weeks after injuring his left knee while playing tennis. Degeneration and partial thickness tearing were found in the tendon. The tendon was debrided, and the calcific deposits were removed. The anterior and posterior layers of the tendon were sutured together.
The longitudinal tear in the tendon was repaired using side-to side-suturing.
The patient was allowed to weight-bear as tolerated two weeks after surgery. Physical Therapy facilitated rapid return to full range of motion. By 8 weeks postsurgery, he was able to walk 5 miles and ride a stationary bike without limitations. His calcific deposits have not recurred.

Discussion
Calcific tendonitis of the patellar tendon is an uncommon condition that has not been well documented thus far [5,6]. X-ray, MRI, and ultrasound are often used in evaluating knee injuries, however these imaging modalities and protocols may be insufficient in diagnosing calcific tendonitis.
Calcific tendonitis is typically seen as a loss in signal intensity on standard MRI sequences, however these sequences pose diagnostic difficulty due to the low number of resonating protons in calcium deposits [4].
As a result, alternative MRI imaging techniques such as susceptibility weighted imaging (SWI) and gradient echo sequences (GREs), have been considered [7]. Typical MRI sequences consist of spin echo sequences such as T1-weighted sequences and T2-weighted sequences in which tendons are visualized as low signal intensities. GREs, however, visualize tendons with high signal intensities due to shorter echo times in comparison to spin echo sequences [8].
GREs are particularly useful in detecting calcific tendonitis as these sequences reveal positive phase shifts which are absent on standard MR imaging [9].
As a result, these imaging techniques can be used to detect calcific tendonitis which is not seen on x-ray, ultrasound, and standard MRI. However, GREs have been shown to be susceptible to signal artifacts [10].

Conclusion
Calcific tendinitis of the patellar tendon is an unusual condition that can present diagnostic difficulty.
Commonly used imaging modalities and protocols may be insufficient in diagnosing calcification.
Although rare, patellar tendon calcific tendonitis may be better visualized with GRE MRI sequences for improved preoperative understanding of treatment and expected outcomes.

Disclosures
The authors have no financial disclosures or conflicts of interest to declare.

Ethical Approval and Consent
Institutional review board approval is not required for a deidentified single case report. Informed consent was obtained from the patient for the publication of this case report.