Hip Joint Pathology as the Primary Cause of Unilateral Lower‑Extremity Spasms

Diagnostic Considerations

• Unilateral muscle spasms, quadriceps (including vastus intermedius) atrophy, hip weakness, and reduced hip range of motion most strongly suggest hip‑joint pathology—specifically femoroacetabular impingement (FAI) syndrome or labral injury【1】.

Pathophysiology

• The abnormal bony morphology of FAI creates premature femoro‑acetabular contact during hip motion, producing mechanical loading that damages chondrolabral structures and directly reduces hip range of motion; this mechanical irritation can provoke reflex spasms in adjacent muscles such as the groin, buttock, quadriceps, and hamstrings【1】.

Exclusion of Non‑Musculoskeletal Serious Conditions

• Prior to confirming a hip‑related diagnosis, clinicians must rule out tumors, infections, stress fractures, and competing lumbar spine pathology, as these serious conditions can mimic the presentation【1】.

Clinical Examination Priorities

• The flexion‑adduction‑internal rotation (FAIR) test: a negative result helps exclude hip‑related pain, although the test’s sensitivity is limited【1】.

Imaging and Interpretation

• Interpretation of hip imaging should be performed by a specialist musculoskeletal radiologist; because both clinical examination and imaging have limited diagnostic utility when used alone, a combined clinical‑radiologic approach is required to accurately diagnose FAI syndrome【1】.

Femoroacetabular Impingement (FAI): Cited Evidence Summary

Definition and Morphology

• Femoroacetabular impingement syndrome arises from abnormal bony morphology that produces premature contact between the proximal femur and acetabulum during hip motion; cam morphology involves a non‑spherical femoral head, while pincer morphology involves excessive acetabular coverage. 2

Pathophysiological Consequences

• Repetitive mechanical loading from cam or pincer impingement leads to chondrolabral damage and early development of hip osteoarthritis. 3

Prevalence of Anatomical Variants

• Anatomical variations that predispose to FAI are common in asymptomatic populations, indicating that the presence of cam or pincer morphology alone does not constitute disease; clinical syndrome requires symptoms, signs, and imaging correlation. 2