Short stem and Monobloc cup: A solution for individual reconstruction of the hip joint with bone sparing implants

For orthopaedic surgeons the main concerns in total hip arthoplasty are:

• Preservation of bone and soft tissue
• Reconstruction of the individual anatomy of the hip joint
• Osteolysis due to reaction of wear
• Bone resorption due to stress shielding

Mathys’ task was to build innovation based on clinical long-term evidence as well as finding a solution to the surgeons’ demand today. This is our answer: optimys – RM Pressfit vitamys.

optimys

The optimys hip stem is the ideal solution for young and active patients but also suitable for older patients with good bone quality. It is designed to reconstruct the individual anatomical conditions of the patient. Extensive X-ray and CT analyses have been performed to determine the optimal shape of the stem, especially at the calcar arch. Due to the distinctive curvature the optimys stem may be used for almost all primary cases. Studies could prove that the stem can either be used in varus or valgus hip, while restoring the centre of rotation offset and leg length (see Figure 1).

Each point on Figure 1 represents a short stem, which is available on the market today. All short stems where compared regarding offset and leg length discrepancy towards the healthy side from three different hip joints. Three stems, which are inside the circle, reproduced the natural anatomy of the patient best. One of them is optimys.

With this solution the surgeon has an option to restore the anatomy of most of his patients. In addition to the bone sparing surgical technique, the stem offers the opportunity to safe as much bone as possible in the trochanteric region. The curved design of the stem provides a reduced stress shielding compared to a conventional straight stem. Biomechanical tests proved this assumption.

The proximal femoral strain pattern was more physiological after insertion of an optimys prosthesis.¹

RM Pressfit vitamys: the innovative solution on the acetabular side

Osteolysis as a consequence of the biological reaction to UHMW-PE wear and metallic debris is the major failure mode of cemented and uncemented acetabular fixation.² It is known that UHMW-PE wear is negatively correlated with the long-term survival of both, cup and stem,³ and that wear rates of standard UHMW-PE are strongly affected by ageing (oxidation).

Therefore a reduction in wear over time is the future challenge for uncemented hip prosthesis.⁴ A promising solution to future challenge of wear reduction is the RM Pressfit vitamys. vitamys enables a better wear resistance (see Figure 2).

Wear rate of vitamys

• is 80% lower than the standard UHMW-PE⁵
• remained at a constant low level over the years (in vitro)⁵
• remain at a constant level using different head materials (CoCr/ceramic) and diameters (28mm, 36mm)⁵

The problem of stress shielding is not only a concern on the stem side. Our approach to these questions is based on the RM philosophy. The elasticity of the UHMW-PE body of the cup provides the elastic behaviour to the acetabular bone. This enables the RM Pressfit vitamys cup to absorb micromotions in the pelvis and prevent stress shielding.

The modulus of elasticity of UHMW-PE and vitamys is very similar to that of human bone (see Table 1).

Conclusion

optimys – to restore the individual anatomical condition, while preserving as much bone as possible.
RM Pressfit vitamys – a promising solution to reduced wear and prevent stress shielding.

References

1. Bieger et.al (2013) Biomechanics of a Short Stem: In Vitro Primary Stability and Stress Shielding of a Conservative Cementless  Hip Stem ,J Orthop Res. 2013; Epub Mar 28
2. Macheras G, Kateros K, Kostakos A, et al. Eight- to ten-year clinical and radiographic outcome of a porous tantalum monoblock acetabular component. J Arthroplasty 2009;24(5):705
3. Ilchmann T, Markovic L, Joshi A, et al. Migration and wear of long-term successful Charnley total hip replacements. J Bone Joint Surg Br 1998;80-B(3):377
4. Harris WH. Conquest of a worldwide human disease: particle-induced periprosthetic osteolysis. Clin Orthop 2004; 429:39-42
5. Beck M, Delfosse D, Lerf R, Becker R, French G, Hollmann L, Knahr K, Naeder H, Meijer E, Rehbein P, Baines J, Steenmeijer A, de Gast A. Oxidation prevention with vitamin E in a HXLPE isoelastic monoblock pressfit cup: Preliminary results in Knahr K (Ed.), Total Hip Arthroplasty, Springer Press, 2012.

Staff Writer