PER-OPERATIVE VIBRATION ANALYSIS: A VALUABLE TOOL FOR DEFINING CORRECT STEM INSERTION

Abstract

The operation technique and prosthetic materials for total hip replacement (THR) have continuously improved. Still, defining the end-point of the prosthetic stem insertion into the femur canal relies on the feeling of the orthopaedic surgeon. This consists of a sense of mechanical stability when exerting torque forces on the prosthesis as well as a feeling of the prosthesis being well fixed and not displaceable along the axis of the femur. Stability and survival of the implant is directly related to the long term fixation stability of the prosthesis stem. But, excessive press-fitting of a THR femoral component can cause intra-operative fractures.

In our centre custom made stem prostheses are commonly used to increase the optimal fit in the femoral canal. We report the first per-operative use of a non invasive vibration analysis technique for the mechanical characterization of the primary bone-prosthesis stability.

From in vitro studies a protocol has been derived for per-operative use. The prosthesis neck is attached to a shaker using a stinger provided with a clamping system. The excitation is realized through white noise in the range 0–12.5 kHz, introducing a power of approximately 0.5W into the femur-prosthesis system. The input force and the response acceleration are measured in the same point with an impedance head mounted between the shaker and the stinger. The Frequency Response Function (FRF) is measured and recorded by a Pimento vibration analyzer connected to a portable computer provided with the appropriate software. All equipment is installed in the surgical theatre but outside the so-called laminar flow area.

The surgeon inserts the implant in the femoral canal through repetitive controlled hammer blows. After each blow, the FRF of the implant-bone structure is measured directly on the prosthesis neck. The hammering is stopped when the FRF graph does not change noticeably anymore.

The amount of FRF change between insertion steps is quantified by the Pearson’s correlation coefficient R between successive FRFs. A correlation between the FRFs of successive stages of R=(0.99 +/− 0.01) over the range 0–10000 Hz is proposed as an endpoint criterion.

Non-cemented custom made stem insertion was studied in 30 patients. In 26/30 cases (86.7%), the correlation coefficient between the last two FRFs was > 0.99 when the surgeon stopped the insertion. In 4 cases, the surgeon decided to stop the insertion because of suspected bone fragility, the final correlation coefficient was lower.

In one case an abnormal change in the FRF graph triggered inspection of the femur bone. A small fracture was observed and insertion was stopped.

In a second case FRF graph showed an oscillating behaviour, while the stem was visibly not completely inserted. After withdrawal of the stem and readjustment of the femoral canal, the stem could be reinserted and the Pearson’s correlation index at end of insertion was 0.998.

The use of custom made stem prosthesis, made exactly to fit into the femoral canal increases the risk of excessive press fit and intra-operative fractures. Vibration analysis showed to be a useful tool to define end of the stem insertion.