We have developed a technique with the underlying principle being that the difference in height between what is removed and what is inserted will determine the leg length correction (Figure 1). The height of the implant to be inserted is determined from the manufacturer's specifications. We have developed a Vertical Measurement Tool to accurately and reproducibly determine the height of the resected bone (Figure 2).

Leg length correction = a−b−c+d

Vertical Measurement Tool validation was performed by 4 separate surgeons on 20 resected femoral heads in the laboratory. Inter and intra-observer error was assessed. Fifty patients were assessed clinically and radiologically, to assess if desired leg length correction was achieved.

Statistical analysis showed the device to be accurate with high intra and inter observer reliability. Differences between the observers were tested using a general linear model in a repeated measure design. No main effect and interaction effects were found. Intra operatively the resected head was measured and the formula was applied. The range of desired correction was 0mm to 18mm. In all cases the post-operative correction was within 4mm of the pre-operative planned correction. Statistical analysis showed that a linear Regression with ‘Actual’ as dependent and ‘Lambda’ as independent variables resulted in R= 0.889.

We believe that it is consistently possible to achieve a leg length correction to within 5 mm of the pre-operative plan using the Vertical Measurement System. The system is simple and reproducible even in the hands of relatively inexperienced surgeons.

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