Biomechanical advantages of a short hip stem

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Over the past two decades, short, cementless hip stems have increasingly been used in total hip arthroplasty (THA). The design concept of these short (hip) stems promises better bone and soft-tissue preservation without negatively affecting implant survival. What is the biomechanical basis for such promises? Do short stems of different design concepts, such as the geometry, materials, and surface finishing, differ in their biomechanical properties? How does the length of the stems affect their biomechanical properties? In this article, Marco Ezechieli (chief orthopedic surgeon at the Clinic for Orthopedics, Traumatology, and Sports traumatology, St Vincenz Hospital Ltd. Paderborn, location St. Josef's Hospital, Salzkotten, Germany) reviews the current evidence on the biomechanical advantages of short stems over traditional designs.

 

Are short stems in total hip arthroplasty really necessary?

Total hip arthroplasty is one of the most popular and successful surgeries that leads to reduced pain and improved function and quality of life for patients. It is also cost-effective with a high implant survival rate [1–3]. However, the indication for THA has been widening and it has been applied in increasingly younger patients. Although national registries have shown that the 10-year implant survival rate for patients older than 75 years is about 98%, it is about 88% for patients younger than 50 years [4]. Bayliss et al [2] showed an even more concerning trend: When analyzed according to the age of the patient at the time of primary THA, the lifetime risk of implant revision (LTRR) for patients 75 years or older was about 4% or lower (Figure 1). For men 50–54 years of age, however, the LTRR was about 30% [2]. Such results caution us that patients who receive their primary THA at a younger age are more likely to require a revision surgery during their lifetime—not just because they have a longer life expectancy, but also because they are likely to be more active, and therefore, implants in young patients will have more cycles of usage and need to sustain higher load in comparison to implants in older patients.

Figure 1. Lifetime risk of surgical revision after total hip arthroplasty [2]. The plot, stratified by gender, shows estimates of lifetime risk of revision against age (in 5-year age bands) at the time of primary surgery. Figure reproduced with permission under the terms of the Creative Commons CC-BY License. Source: Bayliss LE, Culliford D, Monk AP, et al. The effect of patient age at intervention on risk of implant revision after total replacement of the hip or knee: a population-based cohort study. Lancet. 2017 Apr 8;389(10077):1424–1430.

Aiming for bone stock preservation

To address the need of younger and more active patients, short femoral stems were originally developed based on a bone- and soft-tissue-sparing implantation approach, with the primary concept of metaphyseal anchoring [3]. Although numerous short stems have emerged through innovative designs and modifications of standard stems through the years, the most popular short stems currently in use can be roughly divided into two groups based on their anchoring concepts, ie, calcar-loading versus shortened tapered stems [5]. The main goal of a short-stem design is to preserve proximal bone stock and to allow more physiological proximal loading, which should lead to reduced stress shielding. Since stress shielding induces demineralization of the proximal bone, the utilization of short stems in THA promises preservation of proximal bone stock over the long term (see also the section "Reduced stress shielding"). In the event of a revision, the cementless feature can lead to an easier surgery, and the retention of proximal bone stock allows anchoring and osseointegration of the revisional THA stems [1, 4–8].

Other potential advantages of short stems include more anatomical reconstruction of the hip joint [4, 9, 10], as well as preventing impingement of the femoral cortex and thigh pain by avoiding the extension of the stem into the diaphysis [6]. In addition, the modern calcar-guided short stems are inserted by following the calcar of the femoral neck, this allows for an individualized reconstruction based on the patient’s anatomy [3].

Marco Ezechieli

Vincenz Group Paderborn, St Josefs Hospital Salzkotten, Germany

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  • Classification of short stems
  • Biomechanical advantages of femoral neck conservation
  • Key biomechanical considerations: primary implant stability and stress shielding
  • Primary stability: Is it a trade-off for stress shielding?
  • Reduced stress shielding
  • Gruen zones
  • Not all short stems are the same
  • Anatomical reconstruction
  • Potential for further development
  • Conclusion

Part 2 | Midterm results

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Part 3 | Conventional versus short stem

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Contributing experts

This series of articles was created with the support of the following specialists (in alphabetical order):

Martin Buttaro

Italian Hospital Buenos Aires, Argentina

Marco Ezechieli

Vincenz Group Paderborn, St Josefs Hospital Salzkotten, Germany

Carlos Martin Lucero

Italian Hospital Buenos Aires, Argentina

This issue was written by Maio Chen, AO Innovation Translation Center, Clinical Science, Switzerland.

References

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