Biomechanics of a cemented short stem: a comparative in vitro study regarding primary stability and maximum fracture load

Abstract

Purpose: In total hip arthroplasty, uncemented short stems have been used more and more frequently in recent years. Especially for short and curved femoral implants, bone-preserving and soft tissue-sparing properties are postulated. However, indication is limited to sufficient bone quality. At present, there are no curved short stems available which are based on cemented fixation.

Methods: In this in vitro study, primary stability and maximum fracture load of a newly developed cemented short-stem implant was evaluated in comparison to an already well-established cemented conventional straight stem using six pairs of human cadaver femurs with minor bone quality. Primary stability, including reversible micromotion and irreversible migration, was assessed in a dynamic material-testing machine. Furthermore, a subsequent load-to-failure test revealed the periprosthetic fracture characteristics.

Results: Reversible and irreversible micromotions showed no statistical difference between the two investigated stems. All short stems fractured under maximum load according to Vancouver type B3, whereas 4 out of 6 conventional stems suffered a periprosthetic fracture according to Vancouver type C. Mean fracture load of the short stems was 3062 N versus 3160 N for the conventional stems (p = 0.84).

Conclusion: Primary stability of the cemented short stem was not negatively influenced compared to the cemented conventional stem and no significant difference in fracture load was observed. However, a clear difference in the fracture pattern has been identified.

Keywords: Biomechanics; Cadaver; Cemented total hip arthroplasty; Fracture pattern; Micromotion; Migration; Primary stability; Short stem.

Fig. 1

Cemented optimys (left), and twinSys stem (Mathys Ltd., Bettlach, Switzerland)

Fig. 2

The test setup. S1 and S2 demonstrate the locations of the two miniature displacement transducers

Fig. 3

a, b Fracture pattern of periprosthetic fractures induced in both groups. All short stems showed proximal fractures according to Vancouver type B3 (a), 4 out of 6 straight stems showed Vancouver type C fractures (b)

Fig. 4

a, b Anteroposterior radiograph of the periprosthetic femoral fracture with consecutive stem loosening of a cemented short stem (a). Anteroposterior radiograph of the fracture of the femur with a cemented conventional stem (b)