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Cluster analysis as a method for determining size ranges for spinal implants: disc lumbar replacement prosthesis
Dang Lei1, Roger L Holder, Francis W Smith
1School of Engineering, Mechanical Engineering, University of Birmingham, Edgbaston, Birmingham, B15 2TT United Kingdom.
Study Design:
Statistical analysis of clinical radiologic data.
Objective:
To develop an objective method for finding the number of sizes for a lumbar disc replacement.
Summary Of Background Data:
Cluster analysis is a well-established technique for sorting observations into clusters so that the "similarity level" is maximal if they belong to the same cluster and minimal otherwise.
Methods:
Magnetic resonance scans from 69 patients, with no abnormal discs, yielded 206 sagittal and transverse images of 206 discs (levels L3-L4-L5-S1). Anteroposterior and lateral dimensions were measured from vertebral margins on transverse images; disc heights were measured from sagittal images. Hierarchical cluster analysis was performed to determine the number of clusters followed by nonhierarchical (K-means) cluster analysis. Discriminant analysis was used to determine how well the clusters could be used to classify an observation.
Results:
The most successful method of clustering the data involved the following parameters: anteroposterior dimension; lateral dimension (both were the mean of results from the superior and inferior margins of a vertebral body, measured on transverse images); and maximum disc height (from a midsagittal image). These were grouped into 7 clusters so that a discriminant analysis was capable of correctly classifying 97.1% of the observations. The mean and standard deviations for the parameter values in each cluster were determined.
Conclusions:
Cluster analysis has been successfully used to find the dimensions of the minimum number of prosthesis sizes required to replace L3-L4 to L5-S1 discs; the range of sizes would enable them to be used at higher lumbar levels in some patients.