Ossification of Bilateral Sacrotuberous Ligaments: Two Cases Report and Literature Review

Ossification of the sacrotuberous ligament is a rare occurrence in soft tissue, with only 15 cases reported in the past few decades. We reported two cases of bilateral ossification in sacrotuberous ligaments and provided a concise review of the literature on this pathology. Clinical data, radiographic outcomes, and diagnostic and treatment details were obtained. This study aimed to summarize this disease's characteristics and investigate its pathogenesis through a review of literature from the last thirty years. This condition is often incidentally confirmed in elderly males via imagiological examination or gross anatomy and presents a low morbidity rate. Its pathogenesis may be related to stress concentration, excessive intake of element ions, injury repair, and improper operative technique. The majority of patients may not exhibit any clinical symptoms or signs and typically do not require medical interventions. It may be complicated with pudendal nerve entrapment syndrome. The long‐term effects of surgical resection and the most effective treatment approach remain areas for further research.


Introduction
T he sacrotuberous ligament (STL) originates from the posterior superior iliac spine, the lower part of the sacrum, and the upper part of the lateral margin of the coccyx, extending obliquely downward and outward.It crosses behind the sacrospinous ligament and terminates at the medial margin of the ischial tuberosity. 1,2][5][6] Ossification of soft tissue, specifically in STL, is an exceptionally rare occurrence.The first case was reported by Gruber in 1876, with bilateral ossification in an elderly male. 7Because the STL is a crucial structure in maintaining pelvic stability, its ossification or dysfunction may be a rare and even undiscovered cause of pain or discomfort in the lumbosacral, perineal, and buttock regions.This poses certain challenges in our clinical practice and could potentially serve as a new direction or topic for research among orthopedic surgeons.The current cases were reported and reviewed in terms of the general characteristics, epidemiology, etiology, pathogenesis, clinical manifestations, diagnosis, differential diagnosis, and therapeutic strategies of the disease.

Case 1
A 70-year-old man presented to our clinic, reporting pain in his right hip following a fall.He recalled a similar incident occurring approximately two years prior, where he landed on his buttocks, but did not seek any medical treatment at the time.The patient described experiencing localized pain and discomfort in his hips, exacerbated by prolonged sitting or squatting.During the physical examination, we observed significant tenderness in the bilateral ischial tuberosity regions.Subsequent tests were conducted, yielding the following results.
The anteroposterior and lateral views of the pelvis on X-ray imaging revealed symmetrical strips of high-density shadows following the sacrotuberous ligaments (STLs), each with a smooth, broad-based edge originating from the bilateral ischial tuberosities.These strips, resembling antelope horns, measured approximately 6.8 centimeters on the left and 6.9 centimeters on the right.Three-dimensional computed tomography (CT) scanning of the pelvis showed a structure akin to fish bones.These symmetrically connected structures extended from the bilateral ischial tuberosities, featuring a uniform density that gradually tapered from distal-lateral to proximal-medial regions.The magnetic resonance imaging (MRI) of the STLs was assessed using oblique sagittal (15 degrees) and cross-sectional views with T1 and T2-weighted sequences.The images revealed a belt-shaped area of hypointense signal, accompanied by a platy-shaped area of hyperintense signal in the middle on the T1-weighted sequence.Additionally, the T2-weighted sequence displayed edema signal in the soft tissue at the rear of the right STL (Figure 1).Laboratory tests were conducted, including blood routines, blood biochemistry, rheumatoid factor, antistreptolysin O test, high sensitivity C-reactive protein, erythrocyte sedimentation rate, and human leukocyte antigen B27.The results of these tests showed no abnormalities.The clinical diagnosis for the patient was ossification of the bilateral STLs.
The patient was prescribed celecoxib capsules, to be taken twice daily for a duration of two weeks.In addition, he was advised to apply a towel, warmed to approximately 45 degrees Celsius, to his hips once per day for a week.He was also guided to engage in a targeted exercise regimen aimed at strengthening his biceps femoris muscles.This regimen included exercise such as hamstring curls.Follow-up visits at two weeks, three months, six months, and twelve months revealed a gradual alleviation of the patient's symptoms.However, the tenderness in his hips persisted, and radiological imaging indicated no significant changes in the ossified areas.

Case 2
This case involved a 64-year-old man with no previous history of hip injury or pain.Before his admission to the hospital following a traffic accident, he was in good overall health.The physical examination revealed that his right hip exhibited deformity due to flexion, adduction, and pronation, accompanied by noticeable elastic fixation and tenderness.Laboratory tests were within normal ranges.Radiological evaluations, including X-ray, CT, and MRI, identified a fracture in the right acetabulum and dislocation of the femoral head.The imaging also indicated calcification in the bilateral STLs (Figure 2).The patient received open reduction and internal fixation for the acetabular fracture and femoral head dislocation.Postoperative care included skin traction and initiating early rehabilitation exercises.The ossification of the STLs was not specifically treated.

General Characteristics and Epidemiology
After reviewing the literature on ossification of STL from 1993 to 2022, we collated all related cases (Table 1).In this summary, four cases were identified through clinical findings, and 11 were detected in gross anatomy, with all instances occurring in males.Of these 15 cases, seven were bilateral, and eight were unilateral.Age data were available for 12 cases, while three cases lacked age information.The reported instances originated from different countries.
According to our research, ossification of STL is a rare occurrence involving calcification in soft tissue or heterotopic ossification (HO).Previous reports have not documented any cases in female or juvenile.Typically, it is confirmed through imageological examination in clinical practice or via gross anatomical studies in the laboratory, predominantly occurring in elderly men with a low incidence rate.This condition generally manifests after the age of 65, suggesting a potential degenerative change in the pelvic region.There is some variability in findings concerning unilateral versus bilateral occurrences.While our study observed no significant difference, Yuvaraj et al. reported a less frequent incidence of bilateral cases. 14he ossified ligament is commonly described as having the shape of a slate-pencil, goat's horn or violin string. 6,9It has a broad-based origin and gradually tapered from distallateral to proximal-medial end.The terminal part may be acicular or fasciculate, and the abnormal ligament can vary in length between 1.1 and 7.2 cm, as observed in the study by Prescher and Bohndorf. 8In their research, which involved the examination of 101 cadavers, eight cases of ossification were identified.They have concluded that while this pathology is not a reliable indicator of spinal diffuse idiopathic skeletal hyperostosis (DISH), it is an acquired lesion potentially linked to injury of the STL.In contrast to other study, Haller et al. believed that the typical sign of spinal ligamentous ossification in DISH could also occur in extraspinal locations. 15

Etiology and Pathogenesis
After collecting the occupational data for all cases, it becomes clear that many of the occupations necessitate prolonged sitting, stooping, or hip flexion.This pattern suggests a possible link between the etiology of the condition and specific occupational demands, which may stem from the concentration of stress and local tissue hypoxia due to maintaining a posture for extended durations.
Zhou et al. documented a case of bilateral lesions, diagnosed concurrently as skeletal fluorosis via X-ray testing in 2000, attributed to excessive fluoride intake. 9They elaborated that the characteristic "goat's horns" sign observed in X-ray imaging could also manifest in conditions like ankylosing spondylitis and cadmium poisoning.It is hypothesized that excessive fluoride consumption may influence the body's calcium ion distribution, potentially leading to the calcification of soft tissue including ligaments, muscles, and teeth.
For decades, post-traumatic HO has been regarded as an uncommon complication following central nervous system trauma or an intractable side effect of major surgical procedures. 16,179][20] Nonetheless, post-traumatic or post-operative calcification of the STL remains a rare occurrence.Shaul et al. reported a case illustrating this phenomenon, where calcification was described as a consequence of inflammatory repair and vascular degeneration.Their study included a 42-year-old male taxi driver who experienced ossification of the right STL, leading to hip pain and discomfort after prolonged sitting.This patient had previously undergone an open reduction and internal fixation for a pelvic injury about six months prior. 10Factors The follow-up CT images taken after 12 months indicated that there were no noticeable changes in the ossification areas.
contributing to post-operative HO may include the extent of soft tissue dissection, the presence of residual bone fragments, and hematoma. 18

Clinical Manifestations and Diagnosis
The majority of patients with ossification of STL exhibit no clinical symptoms or signs.The presence and severity of symptoms or signs are influenced by the size and length of the ossified tissue, and the degree of damage or pressure it places on adjacent tissue.Clinical manifestations often include hip pain, discomfort, and palpable tenderness.Unlike HO in other regions, ossification of the STL rarely affects limb and joint mobility.It has been suggested that ossification of the STL could potentially lead to PNE syndrome, causing chronic perineal pain. 6,11,14,21The definitive diagnostic tools for HO are CT and X-ray. 19,20

Differential Diagnosis
It is crucial to distinguish hip pain or discomfort caused by other conditions, such as sciatica, lumbar disc degeneration, piriformis syndrome, sacroiliitis, ischiogluteal bursitis, STL injury, hematoma, and bone tumor.Notably, ischiogluteal bursitis, once known as "weaver's bottom," shares similar clinical features.However, in ischiogluteal bursitis, a mass at the ischial tubercle is often palpable, and X-ray or CT scanning typically shows no positive findings.Therefore, further diagnostic procedures like MRI and B-ultrasound are often necessary for accurate diagnosis. 22,23erapeutic Strategies Therapeutic strategies for ossification of the STL have been reported.The NSAIDs may be prescribed to alleviate symptom such as pain.While radiotherapy and bisphosphonates might be effective in preventing HO, current clinical practice lacks sufficient experience and evidence to substantiate this claim.16,17,20 The STL is considered as a continuation of the origin of the long head of biceps femoris.5,14,24 Patients with STL pain can often experience benefits through the injection of local anesthetic, relaxation of biceps femoris, and gluteus maximus contraction training.25,26 Surgical resection was a choice for some cases.19,20   area. 11 It ishypothesized that surgical resection might risk damaging the ligament, potentially leading to pelvic instability.This instability could, in turn, result in secondary pain or movement disorders.The ossification of STL could potentially be an uncommon etiological factor contributing to PNE syndrome.Historically, treatment approaches for PNE syndrome have varied, spanning from conservative measures such as lifestyle modifications, analgesics, and pelvic floor physiotherapy to invasive measures such as anesthetic pudendal nerve block, botulinum toxin injections, surgical decompression, radiofrequency, and neuromodulation therapies.But there has never been a consensus on the diagnosis and treatment of PNE syndrome, which often brings us great challenges in clinical practice.The pulsed radiofrequency technique, known for its minimally invasive, safe, and precise nature, has shown promise in treating PNE syndrome. 27 Therfore, it may be a preferable option for treating the case of ossification of the STL with PNE syndrome.

Conclusion
T he majority of patients with ossification of STL may not require any medical interventions.For those who experience pain or discomfort may achieve positive outcomes through conservative treatment and functional exercise.Furthermore, these patients can maintain a stable state for a long time.Whether surgical resection produces long-term complications and what is the most appropriate treatment need further study.

FIGURE 1 X
FIGURE 1 X-ray, CT, and MRI images of the patient.(A, B) "Antelope's horns" sign on anteroposterior and lateral views of pelvis on x-ray imaging.(C-F) CT showed a shape resembling fish bones.(G-J) MRI revealed on oblique sagittal (15 degrees) and cross-sectional views of T1 and T2.(K-N)

FIGURE 2
FIGURE 2 The patient's imaging data.(A) Anteroposterior view of pelvis on X-ray displayed the injury of right hip joint and ossification of bilateral sacrotuberous ligaments.(B-H) CT scanning revealed typical signs of ossified sacrotuberous ligaments, with ossified signal observed at the attachment of the sacrum in three-dimensional reconstruction.(I, J) Cross-sectional views of T1 and T2 sequences of MRI showed the bilateral sacrotuberous ligaments.(K) Postoperative x-ray image.