Optimizing for Head Height, head Offset, and Canal Fit in a Set of Uncemented Stemmed Femoral Components

When inserting an uncemented hip stem, the objectives are to obtain a close fit of the stem in the canal and anatomic head placement. Our goals were to formulate a set of stems which would satisfy these two objectives, and to test the resulting templates on sequential radiographs of hip replacement patients.

Using 98 cases for which a custom primary hip had been designed, thirteen dimensional parameters for a hip stem were defined, most importantly proximal medial width (PMW), proximal lateral width (PLW), head offset (HOF), head height (HHT), mid-stem diameter (BD), and distal diameter (DD). These parameters were analyzed in 155 patients’ radiographs, and the resulting data were evaluated to obtain the optimal combinations of parameters. A 14–size stem system was defined and evaluated on the AP radiographs of 103 successive hip replacement patients.

For each stem diameter between 11 and 17mm, two pairs of PMW and PLW values, equivalent to ‘varus’ and ‘valgus’ shapes, provided the best fit for the population of radiographs. The template analysis showed that out of 103 cases, 93% of offsets were within 4mm of ideal, while 81% of heights were within 1mm of ideal, and 99% were within 4mm of the ideal. Canal fit was within 1.5mm in the proximal-medial Gruen Zone 7 in 58% of the cases.

The dimensional parameters of the 14 size system enabled the close matching of the important dimensional parameters simultaneously. The error that did occur could largely be corrected by modular heads and by minor canal broaching. By providing two stem shape variations for each stem diameter, our system achieved an accurate head center position while simultaneously obtaining a sufficiently close fit in the canal.