Mini-syMposiuM: Whats neW in hip replaceMent Basic principles

cementless femoral component. note: calcar atrophy.(vii) Current developments in short stem femoral implants for hip replacement surgeryWolfram h Kluge

AbstractBone-saving hip arthroplasty using metaphyseal stems is gaining im-

portance 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 im-

plant design and hip resurfacing. a modern femoral implant should spare

healthy femoral bone during implantation, load the neck and metaphy-

sis 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; metaphy-seal 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 num-ber 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 hydroxy-apatite/calciumphosphate generates safe long-term fixation even in less favourable bone quality.24 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, other-wise 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 4Implant 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 fixa-tion. 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 radiolog-ical 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 fixa-tion 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

Figure 1 intermediate term follow up radiograph of a proximally coated 6 2008 elsevier ltd. all rights reserved.

Mini-syMposiuM: Whats neW in hip replaceMent Basic principlesflares 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.

Mayo Conservative HipConcepts for primary fixation offered by current metaphyseal implants are based on a diversity of biomechanical theories. Multi-point cortical fixation supported by cancellous bone com-pression 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

orthopaeDics anD trauMa 23:1

Figure 2 proxima reproduced by kind permission of Depuy .implantation guide aligning itself in the proximal diaphyseal cav-ity. The Mayo Conservative Hip has shown good results in long-term studies.14 Small variations in positioning of this short stem within the metaphysis can greatly influence the hip joint mecha-nism. 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.

The Thrust Plate prosesthisThe 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 2887 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 alterna-tive to a stemmed endoprosthesis. Jacob et al. implanted 102 Thrust Plates. They state that through the implants 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

Metha prothesisA 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 biomechani-cally correct offset and stem antetorsion with use of a modular neck implant.

The Cut prothesisThe Cut prosthesis (ESKA IMPLANTS) can provide good clini-cal 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 loos-ening, three because of persisting thigh pain, one because of immediate vertical migration, and two because of septic loos-ening. The authors concluded that the medium-term survival is unsatisfactory although the surviving implants had a good clini-cal outcome.47 2008 elsevier ltd. all rights reserved.

Mini-syMposiuM: Whats neW in hip replaceMent Basic principles

orthopaeDics anD trauMa

ithout attention to in hip arthroplasty. nt instability: short-sulted in a relaxed e this problem was then the neck. Hip ortance to not only truct the hip offset. ent have become a

cally favourable off- increase the offset to ream the isthmus t required by offset adequately sized to nd far enough dur-l aspect. The latter

Figure 4 thrust plate

smatter what kind, has been used too often wthe principles and requirements for success Wilson discussed one of the reasons for implaness of the femoral implant neck, which reunstable joint. His recommendation to solvto place the stem in valgus and thereby lengsurgeons have since discovered the vital impadapt the implant neck length but also reconsLeg length concerns following hip replacemmajor medico-legal problem.

Individual reconstruction of a biomechaniset is limited because many implants simplyalong with stem size. It is not recommended of the femur in order to fit a bigger implanconsiderations. On the other hand, a stem create the appropriate offset might not desceing implantation because of its bulky distaimplant inevitably leads to leg lengthening.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.2325 Strain increase in the region of the osteotomy of up to 1440 m/m could be detected for the Cigar and up to 1000 m/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 prosthesisThe Birmingham Mid Head Resection prosthesis (Smith and Nephew Orthopaedics Ltd) is an uncemented short stem prothesis

Figure 3 Mayo conservative hip (Zimmer). recon 23:1 48developed for patients with osteonecrosis involving a large volume of the femoral head. McMinn described improved physiologic prox-imal loading for the implant compared to earlier designs of neck-conserving implants. One-year radio-stereometric analysis showed negligible migration and preservation of femoral neck density.26

The development of new implants, which address bone con-servation, 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 pre-commercial 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 12 mm limit at 2 years, suggesting good stability of the prosthesis.

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

truction of a dysplastic right hip. 2008 elsevier ltd. all rights reserved.

Mini-syMposiuM: Whats neW in hip replaceMent Basic principlesThere is not necessarily an association between the metaphy-seal 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.2931

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 prox-imal 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 intro-duced. 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 facili-tates less invasive surgery. The implant offers the widest range of offsets independent of stem size. Promising initial results on

32

Figure 5 silent hip. reproduced by kind permission of Depuy

international.orthopaeDics anD trauMa 23:1 4

short-term follow up have been reported (Figure 6). 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 diaphy-seal 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 remodel-ling around the ingrown implant appears closer to physiological conditions.

Rasp alignment in short stems can be difficult, because guid-ance 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 trochan-ter. 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,

Figure 6 Fitmore hip (Zimmer).9 2008 elsevier ltd. all rights reserved.

Mini-syMposiuM: Whats neW in hip replaceMent Basic principlesparticularly if the surgeon intends to do a high femoral neck resection. Anteversion of the neck and the physiological proxi-mal femoral bend with its apex towards the posterior metaphysis complicate the initial orientation of the implant within the can-cellous 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 par-ticular 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 plan-ning. 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.

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.

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Practice points

pre-operative planning is essential in order to avoid mal-

position 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 utilizedorthopaeDics anD trauMa 23:1 51 2008 elsevier ltd. all rights reserved.

(vii) Current developments in short stem femoral implants for hip replacement surgeryIntroductionImplant designMayo Conservative HipThe Thrust Plate prosesthisMetha prothesisThe Cut prothesisThe Birmingham Mid Head Resection prosthesis

Leg length and offsetPractical considerationsConclusionReferences