Clinical Review of the Zweymuller Femoral Stem

This review summarizes published literature from a range of reputable sources regarding hip prostheses (stems) utilized currently in cementless Total Hip Arthroplasty. The critical review of published clinical studies shows Zweymuller style (Alloclassic and SL-Plus) stems in all critical characteristics. Since the introduction of cementless total hip arthroplasty in the 1970s, a range of design philosophies for femoral and acetabular components have demonstrated variable clinical success1,3. Recently cementless components have been yielding clinical results on par and in some cases even surpassing their cemented predecessors2,4,6. As a result, cementless THA is gaining in popularity1,7. The short-term results of four of the best cementless femoral components recorded in the Norwegian Arthroplasty Register as described by Havelin et al, included the Corail, lMT, Profile and Zweymuller stems with revision for loosening <1% at 4.5 years which was comparable to cemented counterparts. The Zweymüller stem was introduced to the global market in 19738. Since its introduction the Zweymüller stem has been implanted in over 700,000 patients9 and has undergone minor design updates. The first generation Hochgezogen was a straight stem with a rectangular cross section tapering in the sagittal plane. The stem was forged from titanium alloy (Ti-6Al-4V) with a grit-blasted surface finish. In 1986 the second generation Alloclassic-SL (StepLess) was introduced10. The Alloclassic evolved from the Hochgezogen to taper in both the sagittal and frontal plane and to replace the Vanadium with Niobium in the Titanium alloy due to cytotoxicity concerns11. The SL alludes to the way the stem sizes increase steplessly and proportionally to allow downsizing without sacrificing stability9. The latest generation of the Zweymüller stem, the SL-PLUS has been selected as the predicate for the Signature Pegasus stem. The SL-PLUS differs slightly from the Alloclassic geometrically, with slight modifications to the neck, proximal surface and cross section3,12. The review presents the findings of a literature review conducted to evaluate the clinical performance and survivorship outcomes of the later generations of the Zweymüller stems. From the: 1Joint Implant Surgery & Research Foundation 46 Chagrin Plaza #118, Chagrin Falls, OH 44022 Non-Profit Founded in 1971 www.jisrf.org *Timothy McTighe, Dr. H.S. (hc) Executive Director Declan Brazil, PhD Director of Research 2 Professor of Orthopaedics and Rehabilitation Yale University School of Medicine * Signature Orthopaedics, NSW, Australia 2,3 Board Member and Clinical / Surgical Advisor to JISRF 4, Orthopaedics New England; www.keggiorthosurgery.com Gaining initial and secondary stability is important to the clinical success of a hip stem implant14. The Zweymüller stem gains initial stability both axially and rotationally. The Zweymüller stem is double tapered to gain axial stability [9]. Early subsidence of the stem is frequently reported15,16; however, it stops once the stem contacts cortical bone, and early subsidence of this stem has not been shown to negatively affect the clinical outcome15. For rotational stability the Zweymüller has a rectangular cross section9. Rotational stability is provided according to the ‘square peg in a round hole’ philosophy. The stem is press fit into the intramedullary canal until the corners of the stem contact cortical bone, thus locking it in place9. A combination of the above design features allow initial stability and hence full weight bearing immediately post-operatively4, even in patients with osteoporotic bone9. The initial stability ensures osseo-integration is possible leading to long-term secondary fixation and stability. The Zweymüller stem’s grit blasted surface promotes osseo-integration and rapid secondary stability8 without the risk of coating delamination17. Svehla et al 18 evaluated the pull out strength of small cylindrical implants made of Ti-6Al-4V with 5 different surface finishes (grit-blasted, grit-blasted with HA, Porocoat, Porocoat with HA and smooth) in an ovine model. It was found that the grit-blasted implants had improved pull out strength compared to the smooth implants. Porocoat and HA coating further increased the implant’s pull out strength; however, the study covered a period of only 12 weeks. Longer term clinical follow-ups of the Zweymüller stem with a grit blasted surface show excellent secondary stability as proven by high rates of radiographic osseo-integration6,15,17,19,20 and often lower rates of revision for aseptic loosening than popular cemented stems4,5. Based on clinical and radiological follow-ups the Zweymüller stem is shown to have sufficient immediate and long term stability6. The Zweymüller achieves stability due to a diaphyseal press fit16. As a result, the proximal femur is shielded from compressive stresses thus leading to bone remodeling in accordance with Wolff’s law21. The bone remodeling observed is typically cortical atrophy in the proximal femur and diaphyseal cortical hypertrophy4,12,16,19,22,24. However, the stress shielding is not associated with instability12,19,25,26 or poor clinical outcomes4 and typically stabilizes after two years23. Zweymüller et al 27 investigated the progress of radiolucent lines that tend to be seen around the Zweymüller stem. Based on the radiographic outcomes of 95 patients, he concluded that consistency in radiolucent lines between 6 and 10 years is an indicator for long-term implant survival. Vervest et al [28] used DEXA (Dual Energy X-ray Absorptiometry) technology to examine the bone mineral density in the femur after implantation of a Zweymüller stem. The study of 32 patients that underwent an unilateral hip replacement allowed the contralateral hip to be used as reference. The study found that at 10 years the most notable reductions in bone mineral density were in zones 6 and 7 (calcar region) and zone 2; however, this was not associated with any clinical consequences or radiographic abnormalities. Karachalios et al 22 documented a 10-year prospective, random study in which 80 female patients diagnosed with osteoarthritis were assigned to four groups. Each group had a Zweymüller, Corail, Optifix, or Autophor 900S hip stem implanted. Each group showed the highest bone loss in Gruen Zone 7 (proximal femur) at two years follow-up. After two years the bone loss stabilized and the bone density steadily recovered. The same phenomenon was observed in stems that depend on a proximal HA coating for fixation, however to a lesser extent. In no cases did the stress shielding result in unsatisfactory clinical outcomes. The cause of periprosthetic bone loss is multifactorial, and based on the results of the study the author suggests the clinical and theoretical relevance of stress shielding is overestimated in literature. 42 Reconstructive Review • October 2011 www.jisrf.org It has been hypothesized that adding a proximal HA coating to the Zweymüller stem would reduce proximal bone atrophy by promoting osseointegration. Christ et al 29 and Steens et al21 have evaluated the effectiveness over the medium term of adding a proximal HA coating to the SL-plus stem. Both studies found that the HA coating improved osseo-integration, increased the bone mineral density and reduced the occurrence of radiolucent lines in the proximal femur. Neither study linked the HA coating to improved clinical outcomes; however, the authors agree that a longer term follow-up is necessary to determine if the superior radiographic findings lead to improved clinical outcomes. Periprosthetic osteolysis results in bone loss around an implant and can lead to a loss of stability and eventual revision22. In clinical studies following patients with Zweymüller stem implants, cases of osteolysis were rare, mild, and did not have a clinical relevance6,11,15,20,23. A leading cause of periprosthetic osteolysis is wear debris generated from polyethylene acetabular cup liners. Hip stems with high levels of osseo-integration inhibit the distribution of wear particles distally along the stem; therefore, femoral osteolysis is less prevalent around well osseo-integrated stems30. Stem migration is frequently observed with the Zweymüller stem15,16 as is typical for tapered stems. The stem is secured in the femoral canal by pressing against the cortical wall, thus creating compressive stresses at the bone prosthesis interface. Due to the viscoelastic nature of bone, the compressive stress is relieved and the stem subsides further down the femoral canal. The tapered design allows the stem to regain stability after initial subsidence14. As a result, stem subsidence is not an unusual finding with the Zweymüller stem; however, it is typically nonprogressive15 and ongoing subsidence is not observed after the 2nd post-operative year16. The surgical approach for accessing the hip joint is largely based on the surgeon’s preference. The direct lateral5,31,32, anterolateral 4,6,15,31,33,34 and posterolateral34,35 approaches to implanting the Zweymüller stem have been reported in clinical literature. Many surgeons have developed less invasive mini-incision approaches to implant the Zweymüller stem25,36,39; however, with the large lateral trochanter flair insertion in a single direct anterior approach can be very difficult requiring more posterior soft tissue releases. The surgeon must be aware of the consequences of their chosen surgical approach. The muscular trauma endured during the procedure may lead to redistribution of muscle forces and subsequent bone remodeling. Perka et al 40 showed that the transgluteal approach leads to significantly lower bone mineral density in the proximal femur when compared to the anterolateral approach. The Zweymüller stem uses a “fit without fill” surgical technique. The intramedullary canal is prepared by impacting the cancellous bone using a broach by this technique. In contrast, many competing cementless stems use a “fit with fill” surgical technique in which the intramedullary canal is prepared by clearing its contents. The “fit without fill” technique boasts many advantages over the latter technique, including preserving endosteal blood supply, improving initial stability and fitting a variety of bone shapes9. The endosteal blood supply is preserved because the contents of the intramedullary canal are less disrupted by the Zweymüller surgical technique. Hence the Zweymüller stem can gain initial stability in a wide variety of femoral bone shapes because the canal is broached to the size of the stem, as opposed to the “fit with fill” technique where the stem depends on fitting the irregularly shaped femoral canal for stability. In 1998, Bourne et al41 established an algorithm for deciding whether a cementless or cemented stem should be used, based on experience and a review of the current clinical literature. They suggest that cementless stems should be used in patients younger than 75 years with Dorr type A or B bone shapes and good quality bone stock. Bourne et al suggest that patients older than 75 years with cylindrical type C bone and poor bone stock are better suited to cemented hip replacement. Many surgeons employ this philosophy. Delaunay et al 34 avoided using the www.jisrf.org Reconstructive Review • October 2011 43 Joint Implant Surgery & Research Foundation