Executive Summary: The 21-Year Diagnostic Trap
This analysis examines the critical case report “An Unusual Cause of Locking,” which narrates a profound diagnostic failure that plagued a patient for over two decades. Following an initial penetrating injury, an adult male suffered 21 years of chronic, intermittent mechanical knee locking—the classic symptom of a physical obstruction. Despite these compelling clinical signs, standard radiological investigations, including repeated plain X-rays and advanced Magnetic Resonance Imaging (MRI), were consistently reported as normal.
The elusive cause, ultimately confirmed through diagnostic arthroscopy, was a mobile, intra-articular plastic foreign body. Its safe removal immediately resolved the patient’s long-standing symptoms. This case offers a crucial orthopaedic lesson: clinical suspicion, driven by a patient’s mechanical history, must always override negative imaging results, especially when the presence of radiolucent material is plausible.
The Diagnostic Dilemma: When Technology Fails
True mechanical locking—the sudden physical inability to achieve full knee extension—is typically caused by common pathologies like displaced meniscal tears or osteochondral “loose bodies” (joint mice). These fragments are usually detectable by conventional imaging.
However, the plastic foreign body presented a unique challenge as an exogenous, radiolucent material. The failure of plain X-ray stemmed from basic physics: plastic’s density closely mimics that of surrounding soft tissue and synovial fluid, failing to absorb X-rays distinctly enough to register contrast on a standard film.
Similarly, the highly sensitive MRI proved unreliable. Inert polymer plastics often produce a very low signal, making them difficult to distinguish from background noise, particularly since this fragment had resided within the joint for years without inciting a strong, reliable inflammatory marker (synovitis) that might alert the radiologist. The resulting two-decade delay in diagnosis confirms the danger of “imaging bias” when managing trauma involving non-metallic foreign bodies.
A Refined Protocol and Definitive Intervention
To prevent similar failures, the standard diagnostic algorithm for penetrating knee trauma must be immediately refined if the initial X-ray is negative. Investigation should rapidly transition to modalities specifically designed to detect radiolucent objects:
- Ultrasound (US): Highly effective for superficial and joint foreign bodies, as plastic and wood are typically highly echogenic, creating a distinct hyperechoic signal and posterior acoustic shadow.
- Computed Tomography (CT): Superior for deeper localization, as it detects minute differences in tissue density, surpassing X-ray sensitivity.
Ultimately, when chronic, true mechanical locking persists—as was the case here—surgical exploration is mandatory. Arthroscopy remains the gold standard for the diagnosis and extraction of loose bodies from the knee joint, offering a minimally invasive, curative solution. This case vividly highlights that a vigilant, symptom-driven approach, supported by specialized imaging, is essential to mitigate long-term patient morbidity associated with retained foreign objects.
Professor Wilson’s Comments
“This was one of the earlier papers I wrote and it remains a memorable case. It came as a great surprise to both me and the patient when we discovered — and removed — a large piece of plastic from his knee that had been causing intermittent locking for over 21 years.
Despite numerous assessments and multiple imaging studies over the years, the true cause of his symptoms had gone undetected. Ultimately, it was only through arthroscopy that we were able to identify and remove the offending fragment, bringing an end to his long-standing problems.
This case serves as an important reminder that, while modern imaging is highly reliable, there is always a margin of error. As clinicians, we must remain vigilant and, when appropriate, undertake direct surgical evaluation — such as arthroscopy — to gain a first-hand understanding of the problem and provide definitive treatment.”
Link to the full paper: An Unusual Cause of Locking
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