Influence of undersized cementless hip stems on primary stability and strain distribution

Area of Science:

• Orthopedic biomechanics
• Biomaterials science
• Surgical implant technology

Background:

• Undersizing of cementless hip stems is a known risk factor for aseptic loosening and early subsidence.
• Clinical observations suggest a correlation between suboptimal stem fit and implant failure.

Purpose of the Study:

• To evaluate the biomechanical effects of undersized cementless hip stems.
• To determine if in vitro biomechanical studies can predict clinical outcomes related to stem undersizing.

Main Methods:

• Three sizes of a CLS Spotorno stem were implanted into composite femora.
• Canal Fill Index (CFI) determined stem sizing (undersized <80%, appropriate >80%).
• 3D-micromotions and surface strains were measured to assess primary stability and stress shielding.

Main Results:

• Undersized stems exhibited significantly higher micromotions compared to appropriately sized stems.
• The highest micromotions were observed at the distal stem tip.
• Surface strain analysis showed a tendency for proximal stress shielding with all stem sizes.

Conclusions:

• This study confirms that undersized cementless hip stems lead to significantly reduced primary stability.
• In vitro biomechanical testing is a valuable tool for assessing the effects of undersizing and stress shielding in hip implants.