Single photon emission computed tomography in detection of anterior cruciate ligament tear

Sir,

Anterior cruciate ligament (ACL) tear is a frequent sports injury. Magnetic resonance imaging (MRI) is generally accepted as an excellent modality for assessment of soft tissue injuries of knee[1] but false positive studies have been well documented.[23] Reduced accuracy in ACL injuries is described particularly with reference to partial tear.[4] We report a 27-year-old male with undiagnosed chronic left knee pain and inconclusive MRI referred to us for skeletal scintigraphy. Regional knee three phase (vascular, soft tissue and skeletal phase) Tc99m-Methylene diphosphonate (MDP) scintigraphy [Figure 1] showed focal increased vascularity and significant soft tissue tracer uptake in medial aspect of left knee joint. Single-photon emission computed tomography (SPECT)/CT [Figure 2] of knee joints showed focal hot spot in medial condyle of left femur corresponding to the femoral insertion site of ACL. Repeat MRI [Figure 3] showed high signal intensity involving mid substance ACL with disruption of fibers suggestive of ACL tear with anterior translation of tibia. Further, patient underwent arthroscopic examination [Figure 4] and the retracted stump of the native ACL was visualized.

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Figure 1 15 mCi of 99m Tc-MDP (Methylene diphosphonate) was injected intravenously. Immediate vascular, soft tissue phase images of bilateral knees and delayed skeletal phase images were acquired using high-resolution collimators on a dual head variable angle gamma camera. Vascular, soft tissue skeletal phase images show focal increased vascularity and soft tissue tracer uptake in medial aspect of left knee joint

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Figure 2 SPECT CT of knee joints show focal hot spot in medial condyle of left femur corresponding to the femoral insertion site of anterior cruciate ligament

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Figure 3 Repeat MRI of knee joints show high signal intensity involving mid substance anterior cruciate ligament with disruption of fibers suggestive of left anterior cruciate ligament tear. Mild anterior translation of left tibia also noted

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Figure 4 Arthroscopy shows the retracted stump of the native ACL 6 weeks after injury

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Bone scan is a highly sensitive investigation, readily available, and can image entire skeleton at a reasonable cost. Therefore, bone scan remains to be popular despite technological advances in MRI. SPECT/CT is performed for better anatomical delineation. After intravenous injection, Tc99m MDP rapidly distributes to extracellular fluid (ECF) and chemisorbed into the bone. Tc99m MDP accumulates primarily in relation to osteogenic activity. Amorphous calcium phosphate may account for extra osseous soft tissue Tc99m MDP uptake. MRI is generally accepted as an excellent modality for assessment of soft tissue injuries of knee[5] but false positive studies have been well documented.[46] Reduced accuracy in ACL injuries is described particularly with reference to partial tear.[7] However, arthroscopy is an operator dependent procedure and false negatives are described.[68] Therefore, a true gold standard does not exist, leaving room for complementary imaging methods. Although numerous planar and SPECT bone scan studies have been reported on meniscal tear,[12] its utility in ACL tear stands relatively unexplored. Not surprisingly, increased resolution of SPECT has allowed more lesions to be detected in comparison to planar imaging. Additionally, the ability of SPECT to locate the site of other soft tissue and bone injury in association with ACL tear like posterior cruciate ligament (PCL) tear, medial or lateral meniscus tear also has been observed. This may prove to be a valuable asset in detecting unsuspected associated soft tissue injuries. SPECT bone scintigraphy is a sensitive examination in chronic ACL tear and may be an alternate examination in suspected ACL injury particularly when MRI is unavailable/equivocal or if clinical signs are not consistent with ACL tear.