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Does BMI Affect Accuracy of MRI for Knee Dislocation?

March 24, 2009
Written by: , Filed in: Musculoskeletal Radiology
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Does BMI Affect Accuracy of MRI for Knee Dislocation?

A recent study in AJR examined the incidence and cause of knee dislocation in normal and obese patients with evaluation of MRI findings. It also sought to determine if specific injury patterns are associated with an increased body mass index (BMI).

The result have show that although diagnostic accuracy of MRI does not suffer as a result of a high BMI, there is an increased prevalence of popliteal tendon ruptures in obese patients with knee dislocation.


Methodology

Over a 7-year period, patients who were given the diagnosis of knee dislocation at a level 1 trauma center were identified. Two radiologists independently reviewed the MRI results.

Findings were noted, including injury to the anterior cruciate ligament (ACL), posterior cruciate ligament, medial collateral ligament, lateral collateral ligament, popliteus tendon, quadriceps tendon, patellar ligament, and medial retinaculum.

Injuries to the bone, cartilage, menisci, nerves, and arteries were also recorded.

Equivocal cases were resolved by consensus.

Images were classified subjectively as diagnostic or nondiagnostic.

BMI and mechanism of injury were obtained from the patients’ files. MRI findings of those with a BMI >25 were compared with those with a BMI <25, and statistical analysis was performed.

MDCT and MR Update: Body and Musculoskeletal Imaging

University of California San Francisco, Department of Radiology and Biomedical Imaging
MDCT and MR Update: Body and Musculoskeletal Imaging program features helical multi-detector row CT (MDCT) and MRI as it applies to diseases in the chest, abdomen and pelvis, and musculoskeletal system. Newer and established cross-sectional methods are discussed. Click here to read more or order:

MDCT and MR Update: Body and Musculoskeletal Imaging

Results
24 patients were identified as having knee dislocation over the 7-year period. The incidence based on the local population was determined to be 2.3 knee dislocations per million.

Of the 24 patients, 19 had an MRI of diagnostic quality. The mechanism of injury was determined as follows:

  • 7 cases secondary to high-energy trauma
  • 7 cases secondary to athletic injury
  • 10 cases secondary to a simple fall.

When comparing those with a BMI >25 to those with a BMI <25, no statistical significance was found in image quality and subjective diagnostic quality of the MRI study.

The only statistically significant injury pattern identified between both groups was complete rupture of the popliteal tendon, a finding seen only in the overweight group.

In total, 46% of patients in this study with knee dislocation were overweight and underwent low-incidence trauma.

Conclusions
Although diagnostic accuracy of MRI does not suffer due to a high body mass index, there is an increased prevalence of popliteal tendon ruptures in obese patients with knee dislocation.

The annual incidence of low-energy trauma-induced knee dislocations in overweight patients is not insignificant at a level 1 trauma center.

Reviewer’s Comments

These findings can be practically utilized to determine whether knee dislocation occurred in patients with a BMI >25 after spontaneous reduction.

The presence of popliteal tendon rupture in the setting of low-energy trauma in this patient population subset can be reliably used to diagnose those who may have had a recent knee dislocation.

Author: Basil Hubbi, MD

Reference:
Peltola EK, Lindahl J, et al. Knee Dislocation in Overweight Patients. AJR; 2009;192 (January): 101-106

MDCT and MR Update: Body and Musculoskeletal Imaging

University of California San Francisco, Department of Radiology and Biomedical Imaging
MDCT and MR Update: Body and Musculoskeletal Imaging program features helical multi-detector row CT (MDCT) and MRI as it applies to diseases in the chest, abdomen and pelvis, and musculoskeletal system. Newer and established cross-sectional methods are discussed. Click here to read more or order:

MDCT and MR Update: Body and Musculoskeletal Imaging

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