Abstract

The Thompson hemiarthroplasty is a popular hip prosthesis. We present two case reports highlighting a significant alteration in the design of the implant which compromised the success of the operations.

In recent years the manufacturing process of this prosthesis has changed, with a resultant increase in the volume of the stem of 10 ml.

It is essential that manufacturers inform orthopaedic surgeons of any alteration in the design of the implant and supply compatible instrumentation to minimise surgical errors. Surgeons must remain vigilant when checking the compatibility of the trial and definitive prostheses.

The Thompson hemiarthroplasty (Stryker UK Ltd., Newbury, United Kingdom) has remained one of the most popular of hip prostheses since its introduction in the 1950s. One of its strengths is its simplicity, which makes it popular with trainee surgeons and allows a faster operation for the medically-unfit patient.

We present two cases, highlighting a difference in size between the trial and packaged final prosthesis.

Case 1

A 78-year-old woman underwent conversion of a failed dynamic hip screw to a Thompson hemiarthroplasty using an anterolateral approach. The femur was prepared with a box chisel and a Thompson hand reamer (Stryker UK Ltd.). The trial prosthesis was inserted and the hip reduced satisfactorily.

After the introduction of cement a standard prosthesis was opened and inserted into the femur. Despite tapping with a mallet, the neck of the prosthesis remained 1.5 cm proud of the calcar. It was decided to abort the operation and remove the prosthesis and cement while it was soft. The femur was curetted and re-reamed to accommodate the newer Thompson prosthesis which was cemented in the correct position.

Case 2

A 75-year-old woman with a displaced intra-capsular fracture underwent a cemented Thompson hemiarthroplasty.

After reaming, the trial prosthesis was inserted without difficulty. However, after placing the cement, the surgeon was unable to insert the genuine prosthesis fully. Its neck sat 1.5 cm proud of the calcar.

The prosthesis and cement were removed. On a further trial with the definitive prosthesis, it still could not be pushed fully down. The femur was carefully reamed wider and the prosthesis then cemented satisfactorily.

Discussion

The popular Thompson hemiarthroplasty has had only a small number of unfavourable reports of dislocation and acetabular erosion.1,2 We are unaware of any mention in the orthopaedic literature of an alteration to its dimensions. In both cases which we describe, trial reduction was performed using Thompson hemiarthroplasties manufactured by Howmedica. Intra-operatively, it was noted that there was a mismatch between the trial and definitive prostheses which were supplied by the Stryker-Howmedica company (Fig. 1).

Fig. 1a, Fig. 1b

Photographs showing a) a wider anterior spine and b) a wider posterior aspect of the newer prosthesis on the right.

With the formation of Stryker-Howmedica, the manufacturing process altered and the stem of the standard Thompson hemiarthroplasty was increased by 1.5 mm both anteriorly and posteriorly, thereby causing an increased volume of 10 ml, according to Archimede’s principle (Fig. 2). Stryker-Howmedica has analysed the prostheses and agree that there is a disparity between the trial and definitive prosthesis. However, the company state that they did not intentionally alter the design of the prosthesis. They were hitherto unaware of the difference in size so the reamer and trial prosthesis had not been altered to accommodate this change.

Fig. 2a, Fig. 2b

Photographs showing that the lateral spine is a) 11 mm in the newer and b) 9.5 mm in the older prosthesis.

The relatively minor changes to the design would have little clinical significance in patients with wide femoral canals, whose soft cancellous bone would easily accommodate the wider stem. However, our patients had very narrow femoral canals and the increased width of the stem and volume prevented a satisfactory fit, even without cement.

It is essential that manufacturers inform orthopaedic surgeons of any changes to implants and ensure that the instrumentation is compatable. While it is unclear when these alterations to the design took place, our case reports highlight the importance of checking the trial against the definitive prosthesis to prevent similar mistakes.

Footnotes

  • No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article.

  • Received December 20, 2005.
  • Accepted March 2, 2006.

References

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