Neck Sparing Stem Designs Or Metaphyseal Hip Replacements (Stubbies)
The concept of using short stems or ‘stubbies’ is exciting as this is a bone sparing procedure; maintenance of bone stock being vital given that revision hip surgery may be required in the future.
Preclinical tests such as mechanical fatigue tests, finite element analysis and photo-elastic techniques are used to design bone conserving stems that can be effective and have the excellent long term outcomes of some of the conventional femoral stems.
The risk of these short stems is perforation of the outer cortical bone; stress shielding and bone loss, leading to implant loosening or even fracture of the implant.
Historically, surgeons such as Freeman and Pipino identified the mechanical advantage of retaining the femoral neck; however early prototypes were somewhat disappointing in their medium to long-term outcomes.
A number of bone conserving hip joints were developed and used instead of conventional femoral stems. First generation stubbies include Munting, Weber- Huggler Thrust Plate, Stratech, Spiron, Spotorno and the Mayo.
Second generation stems include the Silent, Proxima, McMinn, Proximal Epiphyseal Replacement (PER), and the MSA Total Hip. The MSA Total Hip was designed with a unique lateral T back design to maximise torsional stability and ease of insertion. A modular neck can be utilised with this ‘stubbie’ design.
Stems with design features that address the properties of the metaphyseal area of the femur are considered likely to show good outcomes.