- Email: [email protected]
(vii) Current developments in short stem femoral implants for hip replacement surgery
Mini-symposium: What’s new in hip replacement — Basic principles
(vii) Current developments in short stem femoral implants for hip replacement surgery
Implant design Currently a number of metaphyseal implants have a straight stem extending into the upper diaphysis. The question whether these stems guide forces into the metaphysis or switch load distribution towards the upper diaphyseal area has recently been discussed in studies using dual-energy x-ray absorptiometry. Bone mineral density was used as a parameter to evaluate bone redistribution around the prosthesis.6,7 Condensation of bone at the distal part of the proximal metaphysis and the proximal diaphysis indicates that implants achieve early stability and durable biological fixation. However, radiological analysis implies that bone loading might not be as physiological as expected. Kim et al. published a report, which revealed 87% grade 2 stress shielding, and 13% grade 3 loss at the calcar in a distal metaphyseal load bearing stem at mean follow up of 8.8 years (Figure 1).8 Decking et al. confirmed that a decrease in proximal femoral strain seen with conventional hip prostheses corresponds well to the reduction of bone density noted in clinical follow-up studies.9 These radiological findings might not indicate impairment of clinical outcome in the intermediate term, nevertheless they demonstrate that there is room for improvement in stem design.10 Proximal load transfer and the absence of distal stem fixation are essential prerequisites for the best performance of the femoral bone after primary total hip replacement. A stem-less prosthesis which loads both medial and lateral proximal femoral
Wolfram H Kluge
Abstract Bone-saving hip arthroplasty using metaphyseal stems is gaining importance because the number of young patients is on the increase and hip resurfacing is not always indicated. This article outlines the recent developments in short stem hip replacement following the concept of conservative hip implants. The individual decision for use of a particular type of implant remains crucial because a stem for all indications does not exist. Every patient requires thorough pre-operative planning. Short metaphyseal stems attempt to bridge the gap between straight stem implant design and hip resurfacing. A modern femoral implant should spare healthy femoral bone during implantation, load the neck and metaphysis in a near physiological way, construct a biomechanically favourable offset without unduly lengthening the leg and favour less invasive soft tissue handling during implantation.
Keywords bone sparing; conservative implant; less invasive; metaphyseal stem; physiological load
Introduction This article outlines the recent developments in short stem hip replacement which fulfil the concept of conservative hip implants.1 Diaphyseal cancellous bone-saving hip arthroplasty using metaphyseal stems is gaining importance because the number of young patients requiring hip surgery is on the increase and hip resurfacing is not always indicated. Active bone growth into structured bio-inert stem surfaces lined with or without hydroxyapatite/calciumphosphate generates safe long-term fixation even in less favourable bone quality.2–4 Surgical technique and implant characteristics are of paramount importance for superior results in hip replacement surgery.5 Products new to the market take time to find general acceptance. New biomechanical concepts usually require a training period prior to first time use, otherwise future confidence in an implant may be compromised. On the other hand, implants with problematic technology may well make it impossible to achieve good and reproducible results. Short stem hip implants are usually uncemented prosthetic devices. It is important to realize that metaphyseal stems load in defined proximal femoral structures thereby ensuring long-term fixation.
Wolfram H Kluge MD is Consultant Orthopaedic Surgeon, Hon. Senior Lecturer at the University of Leeds, UK.
ORTHOPAEDICS AND TRAUMA 23:1
Figure 1 Intermediate term follow up radiograph of a proximally coated cementless femoral component. Note: calcar atrophy.
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© 2008 Elsevier Ltd. All rights reserved.
Mini-symposium: What’s new in hip replacement — Basic principles
implantation guide aligning itself in the proximal diaphyseal cavity. The Mayo Conservative Hip has shown good results in longterm studies.14 Small variations in positioning of this short stem within the metaphysis can greatly influence the hip joint mechanism. The implant may be unsuitable when there is critically poor cancellous bone quality and/or adverse cortical anatomy. On the other hand, the Mayo Hip offers a medullary bone-sparing solution in complex femoral deformity (Figure 3). The surgeon should pay particular attention to appropriate individual offset reconstruction.
flares not only requires less intramedullary bone to be removed intraoperatively but also preserves proximal bone stock in the longer term.11,12 Biomechanic assessment of cyclic motion in a short stem prosthesis like the Proxima™ implant (Figure 2) can produce similar results to clinically successful shaft implants. The same is true for fracture occuring when load tested on cadaver bone. The reduced system stiffness of a short stemmed implant suggests better physiological load transfer. Sufficiently good bone stock is required when implanting a short stem because higher cyclic motion and migration were observed for femora with poorer bone quality. Santori et al. reported on clinical and radiological results of a custom made ultra-short stem prothesis with proximal load transfer. 1131 hips were followed up at five-years.13 The stem design was based on a fully coated implant with pronounced lateral flare. The implant provided effective initial stability which remained over time and appeared to imitate the loading pattern in the normal proximal femur.
The Thrust Plate prosesthis The Thrust Plate prosthesis utilizes metaphyseal fixation to transmit load forces of the hip directly onto the femoral neck. Early follow up studies have demonstrated favourable outcomes; larger studies have recently become available.15 Karatosun et al. retrospectively evaluated 71 hips (follow up 28–87 months, patient age over 65 years) after Thrust Plate arthroplasty.16 The overall revision rate was 8.4%. After a history of trauma was excluded, the rate for loosening and technical errors decreased to 4.2%. Karatosun et al. put no age limit on the indication for use of the prothesis. Buergi et al. reported radiological and clinical outcomes of 102 third generation Thrust Plate prostheses with a mean follow-up period of 58 months (implant survival according to Kaplan-Meier 98% after 6 years) Figure 4.17 Fink et al. followed up the survival of 214 implants over a period of at least five years.18 Failure rate was 7.0% (nine aseptic and six septic loosening). The authors concluded that a Thrust Plate implant should not be considered as an alternative to a stemmed endoprosthesis. Jacob et al. implanted 102 Thrust Plates. They state that through the implant’s ability to load the medial cortex of the proximal femur, cortical bone in this region can be preserved (survival rate 98%, mean follow up 144 months).19 According to Angin et al. a comparative gait analysis in patients with intramedullary stemmed prostheses and Thrust Plate prostheses did not produce any remarkable differences.20
Mayo Conservative Hip Concepts for primary fixation offered by current metaphyseal implants are based on a diversity of biomechanical theories. Multi-point cortical fixation supported by cancellous bone compression is typically represented by the Mayo Conservative Hip (Zimmer®). The straight double taper leans on the calcar but does not follow an individual calcarcurve. The implant tip acts like an
Metha® prothesis A recent report published by Lazovic provided data about the short stemmed Metha® prosthesis (Aesculap®) in 150 cases.21 The author pointed out that the shape of the proximal femur limits the flexibility of implant positioning in short stems. Therefore, he employed navigation in order to reconstruct a biomechanically correct offset and stem antetorsion with use of a modular neck implant. The Cut prothesis The Cut prosthesis (ESKA IMPLANTS) can provide good clinical and radiological results, but has shown a higher loosening rate compared with cementless standard stems. Ender et al. reported on 123 Cut femoral neck prostheses (average patient age 53 years) after a mean follow-up of five years.22 Thirteen of the implants had been revised, seven because of aseptic loosening, three because of persisting thigh pain, one because of immediate vertical migration, and two because of septic loosening. The authors concluded that the medium-term survival is unsatisfactory although the surviving implants had a good clinical outcome.
Figure 2 Proxima™ reproduced by kind permission of DePuy®.
ORTHOPAEDICS AND TRAUMA 23:1
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© 2008 Elsevier Ltd. All rights reserved.
Mini-symposium: What’s new in hip replacement — Basic principles
Figure 3 Mayo Conservative Hip (Zimmer®). Reconstruction of a dysplastic right hip.
developed for patients with osteonecrosis involving a large volume of the femoral head. McMinn described improved physiologic proximal loading for the implant compared to earlier designs of neckconserving implants. One-year radio-stereometric analysis showed negligible migration and preservation of femoral neck density.26 The development of new implants, which address bone conservation, is often based on experience with clinically proven implants.27 Some of the newer implants however use entirely new concepts. The Silent Hip (DePuy International) for example is a tapered press-fit implant fixed within the femoral neck without contact to the lateral cortex. The implant is currently in the precommercial clinical phase. The developers report that it allows for nearly physiological proximal femoral loading (Figure 5). A pilot clinical and radiological investigation including 41 patients revealed that distal migration of the Silent stem was within a 1–2 mm limit at 2 years, suggesting good stability of the prosthesis.
Stress analysis of various femoral neck implants revealed that the Cigar prosthesis caused the most pronounced changes in stress distribution at the lateral thrust plate around the bored out hole.23–25 Strain increase in the region of the osteotomy of up to 1440 μm/m could be detected for the Cigar and up to 1000 μm/m for the Rip prosthesis. The stress pattern after implantation of the Cut prosthesis remained similar to the pre-interventional femoral stress distribution. The Birmingham Mid Head Resection prosthesis The Birmingham Mid Head Resection prosthesis (Smith and Nephew Orthopaedics Ltd) is an uncemented short stem prothesis
Leg length and offset Wilson remarked in his Report for the Committee for the Study of Femoral-Head Replacement in 1954 that hip prostheses represent a new method of substituting a metallic or plastic counterpart for a portion of skeleton.28 He stated that prosthetic replacement, no matter what kind, has been used too often without attention to the principles and requirements for success in hip arthroplasty. Wilson discussed one of the reasons for implant instability: shortness of the femoral implant neck, which resulted in a relaxed unstable joint. His recommendation to solve this problem was to place the stem in valgus and thereby lengthen the neck. Hip surgeons have since discovered the vital importance to not only adapt the implant neck length but also reconstruct the hip offset. Leg length concerns following hip replacement have become a major medico-legal problem. Individual reconstruction of a biomechanically favourable offset is limited because many implants simply increase the offset along with stem size. It is not recommended to ream the isthmus of the femur in order to fit a bigger implant required by offset considerations. On the other hand, a stem adequately sized to create the appropriate offset might not descend far enough during implantation because of its bulky distal aspect. The latter implant inevitably leads to leg lengthening.
Figure 4 Thrust Plate prosthesis.
ORTHOPAEDICS AND TRAUMA 23:1
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© 2008 Elsevier Ltd. All rights reserved.
Mini-symposium: What’s new in hip replacement — Basic principles
Figure 5 Silent Hip. Reproduced by kind permission of DePuy International. Figure 6 Fitmore™ Hip (Zimmer®).
There is not necessarily an association between the metaphyseal femoral anatomy and neck offset. A large neck offset might present with a very narrow proximal femoral canal (Champagne glass). One could argue that these patients are candidates for hip resurfacing. Indeed this often appears to be the most appropriate treatment option. However, indications for hip resurfacing are limited. Despite promising reports, femoral head vascularity and risks like femoral neck fracture/resorption should be considered individually.29–31 It appears a logical step to remove the defective femoral head and replace it by an implant, which utilises the healthy femoral neck and proximal metaphyseal area for fixation. On-growth to the implant and strengthening of bone should be facilitated by predictable tension/pressure distribution during weight-bearing. The implant must allow for individual offsetreconstruction more or less independent of the stem size avoiding damage to the proximal femoral diaphysis. Implant philosophy has evolved from considering stem alignment in the direction of the diaphyseal axis. Today developers regard it to be safe to fix the stem along the metaphyseal curve. One major advantage of this concept is preservation of the greater trochanter by implantation through the femoral neck. Recently a further metaphyseal stem concept has been introduced. The Fitmore™ Hip (Zimmer®) focuses on reconstruction of individual anatomy as accurately as possible. This anatomical stem follows the metaphyseal curve along the calcar and facilitates less invasive surgery. The implant offers the widest range of offsets independent of stem size. Promising initial results on short-term follow up have been reported (Figure 6). 32
ORTHOPAEDICS AND TRAUMA 23:1
Practical considerations Survival rates of metaphyseal prostheses currently appear to be lower than for cementless standard stems. Nevertheless, metaphyseal implants have the advantage of preserving proximal femoral medullary bone without the need to disrupt the diaphyseal marrow cavity. Should a change of endoprosthesis become unavoidable, a standard stem anchored in the proximal femur can be utilized.33 In vitro studies of short-stemmed femoral implants have shown more initial migration than for conventional stems. The short implants stabilised when cortical contact was achieved or cancellous bone was compacted sufficiently.34 Lower cyclic motion of the short stems indicate better physiological loading of the bone. Not only intra-operative destruction of the proximal femur is comparatively small but also secondary bone remodelling around the ingrown implant appears closer to physiological conditions. Rasp alignment in short stems can be difficult, because guidance provided by the proximal diaphyseal cavity as in longer stems, is missing. For implants with a shoulder, the surgeon might have to open a gully into the cancellous greater trochanter. Otherwise, the implant deviates into varus position during impaction with increased risk of a calcar crack or intra-operative lateral femoral perforation. Pre-operative analysis of a lateral hip film can be very helpful in order to anticipate potential difficulties in stem implantation, 49
© 2008 Elsevier Ltd. All rights reserved.
Mini-symposium: What’s new in hip replacement — Basic principles
particularly if the surgeon intends to do a high femoral neck resection. Anteversion of the neck and the physiological proximal femoral bend with its apex towards the posterior metaphysis complicate the initial orientation of the implant within the cancellous bone. The experienced surgeon will find the correct entry point far enough posteriorly within the femoral neck osteotomy in order to avoid mal-position, mainly when using a limited soft tissue approach. The decision about an individual indication for use of a particular type of implant remains crucial. A stem for all indications does not exist and surgeons are asked to study the anatomy in every case. Every patient requires thorough pre-operative planning. Medico-legal proceedings in relation to hip replacement surgery are a constant reminder of our responsibilities about implant choice and operative technique. As with any implant, metaphyseal prostheses require special training for the first time user in order to avoid potential pitfalls.
9 Decking R, Puhl W, Simon U, Claes LE. Changes in strain distribution of loaded proximal femora caused by different types of cementless femoral stems. Clin Biomech 2006; 21: 495–501. 10 Karachalios T, Tsatsaronis C, Efraimis G, Papadelis P, Lyritis G, Diakoumopoulos G. The long-term clinical relevance of calcar atrophy caused by stress shielding in total hip arthroplasty: a 10-year, prospective, randomized study. J Arthroplasty 2004; 19: 469–475. 11 Santori N, Albanese CV, Learmonth ID, Santori FS. Bone preservation with a conservative metaphyseal loading implant. Hip Int 2006; 16: 16–21. 12 Westphal FM, Bishop N, Pueschel K, Morlock MM. Biomechanics of a new short-stemmed uncemented hip prosthesis: an in-vitro study in human bone. Hip Int 2006; 16: 22–30. 13 Santori FS, Manili M, Fredella N, Tonci Ottieri, Santori N. Ultra-short stem with proximal load transfer: clinical and radiographic results at five-year follow-up. Hip Int 2006; 16: 31–39. 14 Morrey BF, Adams RA, Kessler M. A conservative femoral replacement for total hip arthroplasty. A prospective study. J Bone Joint Surg 2000; 82-B: 952–958. 15 Steens W, Rosenbaum D, Goetze C, Gosheger G, van den Daele R, Steinbeck J. Clinical and functional outcome of the thrust plate prosthesis: short- and medium-term results. Clin Biomech 2003; 18: 647–654. 16 Karatosun V, Unver B, Gunal I. Hip arthroplasty with the thrust plate prosthesis in patients of 65 years of age or older: 67 patients followed 2–7 years. Arch Orthop Trauma Surg. 2007 Nov 6 [Epub ahead of print]. 17 Buergi ML, Stoffel KK, Jacob HA, Bereiter HH. Radiological findings and clinical results of 102 thrust-plate femoral hip prostheses: a follow-up of 2 to 8 years. J Arthroplasty 2005; 20: 108–117. 18 Fink B, Wessel S, Deuretzbacher G, Protzen M, Ruther W. Midterm results of “thrust plate” prosthesis. J Arthroplasty 2007; 22: 703–710. 19 Jacob HA, Bereiter HH, Buergi ML. Design aspects and clinical performance of the thrust plate hip prosthesis. Proc Inst Mech Eng 2007; 221: 29–37. 20 Angin S, Karatosun V, Unver B, Gunal I. Gait assessment in patients with thrust plate prosthesis and intramedullary stemmed prosthesis implanted to each hip. Arch Orthop Trauma Surg 2007; 127: 91–96. 21 Rapp SM. Surgeon finds navigation enhances his accuracy placing short, modular hip stems. Orthopaedics Today International 2008; 11: 8. 22 Ender SA, Machner A, Pap G, Hubbe J, Grasshoff H, Neumann HW. Cementless CUT femoral neck prosthesis: increased rate of aseptic loosening after 5 years. Acta Orthop 2007; 78(5): 616–621. 23 Wieners G, Pech M, Streitparth F, Jansson V, Plitz W. Photoelastic stress analysis of human femurs before and after implantation of different models of femur neck prostheses. Z Orthop Unfall 2007; 145: 81–87. 24 Steinhauser E, Ellenrieder M, Gruber G, Busch R, Gradinger R, Mittelmeier W. Influence on load transfer of different femoral neck endoprostheses. Z Orthop Ihre Grenzgeb 2006; 144: 386–393. 25 Hofmann D, Ecke H, Nietert M, Langhans M. Experimental study supported by the German Research Society: stress at the proximal femur after implantation of different cementless hip prostheses. Langenbeck’s archives of surgery; Springer Berlin/Heidelberg: 1987 vol. 372, pp. 849. 26 McMinn DJW, Daniel J, Pradhan C. A vascular necrosis in the young patient: a trilogy of arthroplasty options. Orthopedics 2005; 28: 945.
Conclusion Short metaphyseal stems attempt to bridge the gap between straight stem implant design and hip resurfacing. Technically a modern femoral implant should: (A) spare healthy femoral bone during implantation, (B) load the neck and metaphysis in a near physiological way, (C) construct a biomechanically favourable offset without unduly lengthening the leg and (D) favour less invasive soft tissue handling during implantation. ◆
References 1 Learmonth ID. Conservative hip implants. Current Orthopaedics 2005; 19: 255–262. 2 Teloken MA, Bissett G, Hozack WJ, Sharkey PF, Rothman RH. Ten to fifteen-year follow-up after total hip arthroplasty with a tapered cobalt-chromium femoral component (Tri-Lock) inserted without cement. J Bone Joint Surg Am 2002; 84: 2140–2144. 3 Kim KI, Klein GR, Sleeper J, Dicker AP, Rothman RH, Parvizi J. Uncemented total hip arthroplasty in patients with a history of pelvic irradiation for prostate cancer. J Bone Joint Surg Am 2007; 89: 798–805. 4 Parvizi J, Sharkey PF, Hozack WJ, Orzoco F, Bissett GA, Rothman RH. Prospective matched-pair analysis of hydroxyapatite-coated and uncoated femoral stems in total hip arthroplasty. A concise followup of a previous report. J Bone Joint Surg Am 2004; 86: 783–786. 5 Hallan G, Lie SA, Furnes O, Engesaeter LB, Vollset SE, Havelin LI. Medium- and long-term performance of 11 516 uncemented primary femoral stems from the Norwegian arthroplasty register. J Bone Joint Surg 2007; 89-B: 1574–1580. 6 Albanese CV, Rendine M, De Palma F, et al. Bone remodelling in THA: a comparative DXA scan study between conventional implants and a new stemless femoral component. a preliminary report. Hip Int 2006; 16: 9–15. 7 Kulkarni M, Wylde V, Aspros D, Learmonth ID. Early clinical experience with a metaphyseal loading implant: why have a stem? Hip Int 2006; 16: 3–8. 8 Kim YH. The results of a proximally coated cementless femoral component in total hip replacement: a five to 12 year follow-up. J Bone Joint Surg 2008; 90-B: 299–305.
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34 Westphal FM, Bishopa N, Honlb M, Hillec E, Püscheld K, Morlocka MM. Migration and cyclic motion of a new short-stemmed hip prosthesis – a biomechanical in vitro study. Clin Biomech 2006; 8: 834–840.
27 Biomet launches hip technologies to address demand for minimally invasive bone-conserving implants. Biomet, Inc. Warsaw, Indiana. Also available at: http://www.biomet.com. 28 Wilson PD. Report of the Committee for the Study of Femoral-Head Replacement. Symposium on Femoral-Head Replacement Prostheses: based on the Prostheses as Printed in the October (1954) Issue of the Bulletin. J Bone Joint Surg Am 1956; 38: 407–420. 29 Amstutz HC, Beaulé PE, Dorey FJ, Le Duff MJ, Campbell PA, Gruen TA. Metal-on-metal hybrid surface arthroplasty: two to six-year follow-up study. J Bone Joint Surg Am 2004; 86: 28–39. 30 Steffen RT, Pandit HP, Palan J, et al. The five-year results of the Birmingham hip resurfacing arthroplasty. J Bone Joint Surg 2008; 90-B: 436–441. 31 Mont MA, Seyler TM, Plate JF, Delanois RE, Parvizi J. Uncemented total hip arthroplasty in young adults with osteonecrosis of the femoral head: a comparative study. J Bone Joint Surg Am 2006; 88: 104–109. 32 Fitmore™ Hip Stem. A new stem for primary THA. Masterclass. Berne Switzerland 21–22 February 2008. 33 Stukenborg-Colsman C. Femoral neck prostheses. Orthopade 2007; 36: 347–352.
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Practice points • Pre-operative planning is essential in order to avoid malposition of the metaphyseal implant • The surgeon should find the correct entry point far enough posteriorly within the femoral neck osteotomy mainly when using a limited soft tissue approach • Short implants achieve best primary stability when cortical contact is achieved and cancellous bone is compacted sufficiently • Should exchange of the metaphyseal stem become necessary, a standard stem anchored in the proximal femur can be utilized
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© 2008 Elsevier Ltd. All rights reserved.
(vii) Current developments in short stem femoral implants for hip replacement surgery
Implant design Currently a number of metaphyseal implants have a straight stem extending into the upper diaphysis. The question whether these stems guide forces into the metaphysis or switch load distribution towards the upper diaphyseal area has recently been discussed in studies using dual-energy x-ray absorptiometry. Bone mineral density was used as a parameter to evaluate bone redistribution around the prosthesis.6,7 Condensation of bone at the distal part of the proximal metaphysis and the proximal diaphysis indicates that implants achieve early stability and durable biological fixation. However, radiological analysis implies that bone loading might not be as physiological as expected. Kim et al. published a report, which revealed 87% grade 2 stress shielding, and 13% grade 3 loss at the calcar in a distal metaphyseal load bearing stem at mean follow up of 8.8 years (Figure 1).8 Decking et al. confirmed that a decrease in proximal femoral strain seen with conventional hip prostheses corresponds well to the reduction of bone density noted in clinical follow-up studies.9 These radiological findings might not indicate impairment of clinical outcome in the intermediate term, nevertheless they demonstrate that there is room for improvement in stem design.10 Proximal load transfer and the absence of distal stem fixation are essential prerequisites for the best performance of the femoral bone after primary total hip replacement. A stem-less prosthesis which loads both medial and lateral proximal femoral
Wolfram H Kluge
Abstract Bone-saving hip arthroplasty using metaphyseal stems is gaining importance because the number of young patients is on the increase and hip resurfacing is not always indicated. This article outlines the recent developments in short stem hip replacement following the concept of conservative hip implants. The individual decision for use of a particular type of implant remains crucial because a stem for all indications does not exist. Every patient requires thorough pre-operative planning. Short metaphyseal stems attempt to bridge the gap between straight stem implant design and hip resurfacing. A modern femoral implant should spare healthy femoral bone during implantation, load the neck and metaphysis in a near physiological way, construct a biomechanically favourable offset without unduly lengthening the leg and favour less invasive soft tissue handling during implantation.
Keywords bone sparing; conservative implant; less invasive; metaphyseal stem; physiological load
Introduction This article outlines the recent developments in short stem hip replacement which fulfil the concept of conservative hip implants.1 Diaphyseal cancellous bone-saving hip arthroplasty using metaphyseal stems is gaining importance because the number of young patients requiring hip surgery is on the increase and hip resurfacing is not always indicated. Active bone growth into structured bio-inert stem surfaces lined with or without hydroxyapatite/calciumphosphate generates safe long-term fixation even in less favourable bone quality.2–4 Surgical technique and implant characteristics are of paramount importance for superior results in hip replacement surgery.5 Products new to the market take time to find general acceptance. New biomechanical concepts usually require a training period prior to first time use, otherwise future confidence in an implant may be compromised. On the other hand, implants with problematic technology may well make it impossible to achieve good and reproducible results. Short stem hip implants are usually uncemented prosthetic devices. It is important to realize that metaphyseal stems load in defined proximal femoral structures thereby ensuring long-term fixation.
Wolfram H Kluge MD is Consultant Orthopaedic Surgeon, Hon. Senior Lecturer at the University of Leeds, UK.
ORTHOPAEDICS AND TRAUMA 23:1
Figure 1 Intermediate term follow up radiograph of a proximally coated cementless femoral component. Note: calcar atrophy.
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© 2008 Elsevier Ltd. All rights reserved.
Mini-symposium: What’s new in hip replacement — Basic principles
implantation guide aligning itself in the proximal diaphyseal cavity. The Mayo Conservative Hip has shown good results in longterm studies.14 Small variations in positioning of this short stem within the metaphysis can greatly influence the hip joint mechanism. The implant may be unsuitable when there is critically poor cancellous bone quality and/or adverse cortical anatomy. On the other hand, the Mayo Hip offers a medullary bone-sparing solution in complex femoral deformity (Figure 3). The surgeon should pay particular attention to appropriate individual offset reconstruction.
flares not only requires less intramedullary bone to be removed intraoperatively but also preserves proximal bone stock in the longer term.11,12 Biomechanic assessment of cyclic motion in a short stem prosthesis like the Proxima™ implant (Figure 2) can produce similar results to clinically successful shaft implants. The same is true for fracture occuring when load tested on cadaver bone. The reduced system stiffness of a short stemmed implant suggests better physiological load transfer. Sufficiently good bone stock is required when implanting a short stem because higher cyclic motion and migration were observed for femora with poorer bone quality. Santori et al. reported on clinical and radiological results of a custom made ultra-short stem prothesis with proximal load transfer. 1131 hips were followed up at five-years.13 The stem design was based on a fully coated implant with pronounced lateral flare. The implant provided effective initial stability which remained over time and appeared to imitate the loading pattern in the normal proximal femur.
The Thrust Plate prosesthis The Thrust Plate prosthesis utilizes metaphyseal fixation to transmit load forces of the hip directly onto the femoral neck. Early follow up studies have demonstrated favourable outcomes; larger studies have recently become available.15 Karatosun et al. retrospectively evaluated 71 hips (follow up 28–87 months, patient age over 65 years) after Thrust Plate arthroplasty.16 The overall revision rate was 8.4%. After a history of trauma was excluded, the rate for loosening and technical errors decreased to 4.2%. Karatosun et al. put no age limit on the indication for use of the prothesis. Buergi et al. reported radiological and clinical outcomes of 102 third generation Thrust Plate prostheses with a mean follow-up period of 58 months (implant survival according to Kaplan-Meier 98% after 6 years) Figure 4.17 Fink et al. followed up the survival of 214 implants over a period of at least five years.18 Failure rate was 7.0% (nine aseptic and six septic loosening). The authors concluded that a Thrust Plate implant should not be considered as an alternative to a stemmed endoprosthesis. Jacob et al. implanted 102 Thrust Plates. They state that through the implant’s ability to load the medial cortex of the proximal femur, cortical bone in this region can be preserved (survival rate 98%, mean follow up 144 months).19 According to Angin et al. a comparative gait analysis in patients with intramedullary stemmed prostheses and Thrust Plate prostheses did not produce any remarkable differences.20
Mayo Conservative Hip Concepts for primary fixation offered by current metaphyseal implants are based on a diversity of biomechanical theories. Multi-point cortical fixation supported by cancellous bone compression is typically represented by the Mayo Conservative Hip (Zimmer®). The straight double taper leans on the calcar but does not follow an individual calcarcurve. The implant tip acts like an
Metha® prothesis A recent report published by Lazovic provided data about the short stemmed Metha® prosthesis (Aesculap®) in 150 cases.21 The author pointed out that the shape of the proximal femur limits the flexibility of implant positioning in short stems. Therefore, he employed navigation in order to reconstruct a biomechanically correct offset and stem antetorsion with use of a modular neck implant. The Cut prothesis The Cut prosthesis (ESKA IMPLANTS) can provide good clinical and radiological results, but has shown a higher loosening rate compared with cementless standard stems. Ender et al. reported on 123 Cut femoral neck prostheses (average patient age 53 years) after a mean follow-up of five years.22 Thirteen of the implants had been revised, seven because of aseptic loosening, three because of persisting thigh pain, one because of immediate vertical migration, and two because of septic loosening. The authors concluded that the medium-term survival is unsatisfactory although the surviving implants had a good clinical outcome.
Figure 2 Proxima™ reproduced by kind permission of DePuy®.
ORTHOPAEDICS AND TRAUMA 23:1
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© 2008 Elsevier Ltd. All rights reserved.
Mini-symposium: What’s new in hip replacement — Basic principles
Figure 3 Mayo Conservative Hip (Zimmer®). Reconstruction of a dysplastic right hip.
developed for patients with osteonecrosis involving a large volume of the femoral head. McMinn described improved physiologic proximal loading for the implant compared to earlier designs of neckconserving implants. One-year radio-stereometric analysis showed negligible migration and preservation of femoral neck density.26 The development of new implants, which address bone conservation, is often based on experience with clinically proven implants.27 Some of the newer implants however use entirely new concepts. The Silent Hip (DePuy International) for example is a tapered press-fit implant fixed within the femoral neck without contact to the lateral cortex. The implant is currently in the precommercial clinical phase. The developers report that it allows for nearly physiological proximal femoral loading (Figure 5). A pilot clinical and radiological investigation including 41 patients revealed that distal migration of the Silent stem was within a 1–2 mm limit at 2 years, suggesting good stability of the prosthesis.
Stress analysis of various femoral neck implants revealed that the Cigar prosthesis caused the most pronounced changes in stress distribution at the lateral thrust plate around the bored out hole.23–25 Strain increase in the region of the osteotomy of up to 1440 μm/m could be detected for the Cigar and up to 1000 μm/m for the Rip prosthesis. The stress pattern after implantation of the Cut prosthesis remained similar to the pre-interventional femoral stress distribution. The Birmingham Mid Head Resection prosthesis The Birmingham Mid Head Resection prosthesis (Smith and Nephew Orthopaedics Ltd) is an uncemented short stem prothesis
Leg length and offset Wilson remarked in his Report for the Committee for the Study of Femoral-Head Replacement in 1954 that hip prostheses represent a new method of substituting a metallic or plastic counterpart for a portion of skeleton.28 He stated that prosthetic replacement, no matter what kind, has been used too often without attention to the principles and requirements for success in hip arthroplasty. Wilson discussed one of the reasons for implant instability: shortness of the femoral implant neck, which resulted in a relaxed unstable joint. His recommendation to solve this problem was to place the stem in valgus and thereby lengthen the neck. Hip surgeons have since discovered the vital importance to not only adapt the implant neck length but also reconstruct the hip offset. Leg length concerns following hip replacement have become a major medico-legal problem. Individual reconstruction of a biomechanically favourable offset is limited because many implants simply increase the offset along with stem size. It is not recommended to ream the isthmus of the femur in order to fit a bigger implant required by offset considerations. On the other hand, a stem adequately sized to create the appropriate offset might not descend far enough during implantation because of its bulky distal aspect. The latter implant inevitably leads to leg lengthening.
Figure 4 Thrust Plate prosthesis.
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© 2008 Elsevier Ltd. All rights reserved.
Mini-symposium: What’s new in hip replacement — Basic principles
Figure 5 Silent Hip. Reproduced by kind permission of DePuy International. Figure 6 Fitmore™ Hip (Zimmer®).
There is not necessarily an association between the metaphyseal femoral anatomy and neck offset. A large neck offset might present with a very narrow proximal femoral canal (Champagne glass). One could argue that these patients are candidates for hip resurfacing. Indeed this often appears to be the most appropriate treatment option. However, indications for hip resurfacing are limited. Despite promising reports, femoral head vascularity and risks like femoral neck fracture/resorption should be considered individually.29–31 It appears a logical step to remove the defective femoral head and replace it by an implant, which utilises the healthy femoral neck and proximal metaphyseal area for fixation. On-growth to the implant and strengthening of bone should be facilitated by predictable tension/pressure distribution during weight-bearing. The implant must allow for individual offsetreconstruction more or less independent of the stem size avoiding damage to the proximal femoral diaphysis. Implant philosophy has evolved from considering stem alignment in the direction of the diaphyseal axis. Today developers regard it to be safe to fix the stem along the metaphyseal curve. One major advantage of this concept is preservation of the greater trochanter by implantation through the femoral neck. Recently a further metaphyseal stem concept has been introduced. The Fitmore™ Hip (Zimmer®) focuses on reconstruction of individual anatomy as accurately as possible. This anatomical stem follows the metaphyseal curve along the calcar and facilitates less invasive surgery. The implant offers the widest range of offsets independent of stem size. Promising initial results on short-term follow up have been reported (Figure 6). 32
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Practical considerations Survival rates of metaphyseal prostheses currently appear to be lower than for cementless standard stems. Nevertheless, metaphyseal implants have the advantage of preserving proximal femoral medullary bone without the need to disrupt the diaphyseal marrow cavity. Should a change of endoprosthesis become unavoidable, a standard stem anchored in the proximal femur can be utilized.33 In vitro studies of short-stemmed femoral implants have shown more initial migration than for conventional stems. The short implants stabilised when cortical contact was achieved or cancellous bone was compacted sufficiently.34 Lower cyclic motion of the short stems indicate better physiological loading of the bone. Not only intra-operative destruction of the proximal femur is comparatively small but also secondary bone remodelling around the ingrown implant appears closer to physiological conditions. Rasp alignment in short stems can be difficult, because guidance provided by the proximal diaphyseal cavity as in longer stems, is missing. For implants with a shoulder, the surgeon might have to open a gully into the cancellous greater trochanter. Otherwise, the implant deviates into varus position during impaction with increased risk of a calcar crack or intra-operative lateral femoral perforation. Pre-operative analysis of a lateral hip film can be very helpful in order to anticipate potential difficulties in stem implantation, 49
© 2008 Elsevier Ltd. All rights reserved.
Mini-symposium: What’s new in hip replacement — Basic principles
particularly if the surgeon intends to do a high femoral neck resection. Anteversion of the neck and the physiological proximal femoral bend with its apex towards the posterior metaphysis complicate the initial orientation of the implant within the cancellous bone. The experienced surgeon will find the correct entry point far enough posteriorly within the femoral neck osteotomy in order to avoid mal-position, mainly when using a limited soft tissue approach. The decision about an individual indication for use of a particular type of implant remains crucial. A stem for all indications does not exist and surgeons are asked to study the anatomy in every case. Every patient requires thorough pre-operative planning. Medico-legal proceedings in relation to hip replacement surgery are a constant reminder of our responsibilities about implant choice and operative technique. As with any implant, metaphyseal prostheses require special training for the first time user in order to avoid potential pitfalls.
9 Decking R, Puhl W, Simon U, Claes LE. Changes in strain distribution of loaded proximal femora caused by different types of cementless femoral stems. Clin Biomech 2006; 21: 495–501. 10 Karachalios T, Tsatsaronis C, Efraimis G, Papadelis P, Lyritis G, Diakoumopoulos G. The long-term clinical relevance of calcar atrophy caused by stress shielding in total hip arthroplasty: a 10-year, prospective, randomized study. J Arthroplasty 2004; 19: 469–475. 11 Santori N, Albanese CV, Learmonth ID, Santori FS. Bone preservation with a conservative metaphyseal loading implant. Hip Int 2006; 16: 16–21. 12 Westphal FM, Bishop N, Pueschel K, Morlock MM. Biomechanics of a new short-stemmed uncemented hip prosthesis: an in-vitro study in human bone. Hip Int 2006; 16: 22–30. 13 Santori FS, Manili M, Fredella N, Tonci Ottieri, Santori N. Ultra-short stem with proximal load transfer: clinical and radiographic results at five-year follow-up. Hip Int 2006; 16: 31–39. 14 Morrey BF, Adams RA, Kessler M. A conservative femoral replacement for total hip arthroplasty. A prospective study. J Bone Joint Surg 2000; 82-B: 952–958. 15 Steens W, Rosenbaum D, Goetze C, Gosheger G, van den Daele R, Steinbeck J. Clinical and functional outcome of the thrust plate prosthesis: short- and medium-term results. Clin Biomech 2003; 18: 647–654. 16 Karatosun V, Unver B, Gunal I. Hip arthroplasty with the thrust plate prosthesis in patients of 65 years of age or older: 67 patients followed 2–7 years. Arch Orthop Trauma Surg. 2007 Nov 6 [Epub ahead of print]. 17 Buergi ML, Stoffel KK, Jacob HA, Bereiter HH. Radiological findings and clinical results of 102 thrust-plate femoral hip prostheses: a follow-up of 2 to 8 years. J Arthroplasty 2005; 20: 108–117. 18 Fink B, Wessel S, Deuretzbacher G, Protzen M, Ruther W. Midterm results of “thrust plate” prosthesis. J Arthroplasty 2007; 22: 703–710. 19 Jacob HA, Bereiter HH, Buergi ML. Design aspects and clinical performance of the thrust plate hip prosthesis. Proc Inst Mech Eng 2007; 221: 29–37. 20 Angin S, Karatosun V, Unver B, Gunal I. Gait assessment in patients with thrust plate prosthesis and intramedullary stemmed prosthesis implanted to each hip. Arch Orthop Trauma Surg 2007; 127: 91–96. 21 Rapp SM. Surgeon finds navigation enhances his accuracy placing short, modular hip stems. Orthopaedics Today International 2008; 11: 8. 22 Ender SA, Machner A, Pap G, Hubbe J, Grasshoff H, Neumann HW. Cementless CUT femoral neck prosthesis: increased rate of aseptic loosening after 5 years. Acta Orthop 2007; 78(5): 616–621. 23 Wieners G, Pech M, Streitparth F, Jansson V, Plitz W. Photoelastic stress analysis of human femurs before and after implantation of different models of femur neck prostheses. Z Orthop Unfall 2007; 145: 81–87. 24 Steinhauser E, Ellenrieder M, Gruber G, Busch R, Gradinger R, Mittelmeier W. Influence on load transfer of different femoral neck endoprostheses. Z Orthop Ihre Grenzgeb 2006; 144: 386–393. 25 Hofmann D, Ecke H, Nietert M, Langhans M. Experimental study supported by the German Research Society: stress at the proximal femur after implantation of different cementless hip prostheses. Langenbeck’s archives of surgery; Springer Berlin/Heidelberg: 1987 vol. 372, pp. 849. 26 McMinn DJW, Daniel J, Pradhan C. A vascular necrosis in the young patient: a trilogy of arthroplasty options. Orthopedics 2005; 28: 945.
Conclusion Short metaphyseal stems attempt to bridge the gap between straight stem implant design and hip resurfacing. Technically a modern femoral implant should: (A) spare healthy femoral bone during implantation, (B) load the neck and metaphysis in a near physiological way, (C) construct a biomechanically favourable offset without unduly lengthening the leg and (D) favour less invasive soft tissue handling during implantation. ◆
References 1 Learmonth ID. Conservative hip implants. Current Orthopaedics 2005; 19: 255–262. 2 Teloken MA, Bissett G, Hozack WJ, Sharkey PF, Rothman RH. Ten to fifteen-year follow-up after total hip arthroplasty with a tapered cobalt-chromium femoral component (Tri-Lock) inserted without cement. J Bone Joint Surg Am 2002; 84: 2140–2144. 3 Kim KI, Klein GR, Sleeper J, Dicker AP, Rothman RH, Parvizi J. Uncemented total hip arthroplasty in patients with a history of pelvic irradiation for prostate cancer. J Bone Joint Surg Am 2007; 89: 798–805. 4 Parvizi J, Sharkey PF, Hozack WJ, Orzoco F, Bissett GA, Rothman RH. Prospective matched-pair analysis of hydroxyapatite-coated and uncoated femoral stems in total hip arthroplasty. A concise followup of a previous report. J Bone Joint Surg Am 2004; 86: 783–786. 5 Hallan G, Lie SA, Furnes O, Engesaeter LB, Vollset SE, Havelin LI. Medium- and long-term performance of 11 516 uncemented primary femoral stems from the Norwegian arthroplasty register. J Bone Joint Surg 2007; 89-B: 1574–1580. 6 Albanese CV, Rendine M, De Palma F, et al. Bone remodelling in THA: a comparative DXA scan study between conventional implants and a new stemless femoral component. a preliminary report. Hip Int 2006; 16: 9–15. 7 Kulkarni M, Wylde V, Aspros D, Learmonth ID. Early clinical experience with a metaphyseal loading implant: why have a stem? Hip Int 2006; 16: 3–8. 8 Kim YH. The results of a proximally coated cementless femoral component in total hip replacement: a five to 12 year follow-up. J Bone Joint Surg 2008; 90-B: 299–305.
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Mini-symposium: What’s new in hip replacement — Basic principles
34 Westphal FM, Bishopa N, Honlb M, Hillec E, Püscheld K, Morlocka MM. Migration and cyclic motion of a new short-stemmed hip prosthesis – a biomechanical in vitro study. Clin Biomech 2006; 8: 834–840.
27 Biomet launches hip technologies to address demand for minimally invasive bone-conserving implants. Biomet, Inc. Warsaw, Indiana. Also available at: http://www.biomet.com. 28 Wilson PD. Report of the Committee for the Study of Femoral-Head Replacement. Symposium on Femoral-Head Replacement Prostheses: based on the Prostheses as Printed in the October (1954) Issue of the Bulletin. J Bone Joint Surg Am 1956; 38: 407–420. 29 Amstutz HC, Beaulé PE, Dorey FJ, Le Duff MJ, Campbell PA, Gruen TA. Metal-on-metal hybrid surface arthroplasty: two to six-year follow-up study. J Bone Joint Surg Am 2004; 86: 28–39. 30 Steffen RT, Pandit HP, Palan J, et al. The five-year results of the Birmingham hip resurfacing arthroplasty. J Bone Joint Surg 2008; 90-B: 436–441. 31 Mont MA, Seyler TM, Plate JF, Delanois RE, Parvizi J. Uncemented total hip arthroplasty in young adults with osteonecrosis of the femoral head: a comparative study. J Bone Joint Surg Am 2006; 88: 104–109. 32 Fitmore™ Hip Stem. A new stem for primary THA. Masterclass. Berne Switzerland 21–22 February 2008. 33 Stukenborg-Colsman C. Femoral neck prostheses. Orthopade 2007; 36: 347–352.
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Practice points • Pre-operative planning is essential in order to avoid malposition of the metaphyseal implant • The surgeon should find the correct entry point far enough posteriorly within the femoral neck osteotomy mainly when using a limited soft tissue approach • Short implants achieve best primary stability when cortical contact is achieved and cancellous bone is compacted sufficiently • Should exchange of the metaphyseal stem become necessary, a standard stem anchored in the proximal femur can be utilized
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