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abstract

Current Concepts, Classifi cation, and Results in Short Stem Hip ArthroplastyFrancesco Falez, MD; Filippo Casella, MD; Matteo Papalia, MD

Various short hip stems have been introduced with differing implant concepts of femo-ral fi xation and implant length. There is a lack of proper classifi cation for short hip stems, with a clear and accepted defi nition for implant length and extent of bone preservation in the metaphyseal and diaphyseal femur. This study analyzed the length of short hip stems. Stems were divided into collum, partial collum, and trochanter-sparing implants. An additional category was added, trochanter harming, which was defi ned as interruption of the circumferential integrity of the femoral neck. For all of the femoral components described, the designs were compared, excluding stems with insuffi cient clinical data. The 15 fi nally selected stems were classifi ed as collum (1 stem), partial collum (7 stems), trochanter sparing (4 stems), and trochanter harming (3 stems). Mid-term results (>5 years of follow-up) were available for only 3 designs in the partial collum group. Taking into account the results of short-term studies (<5 years of follow-up), the femoral revision rate per 100 observed component years was <1 for most total hip arthroplasties. However, the studies varied greatly regarding level of sig-nifi cance, and short hip stems without published results are available commercially. Short hip stems cannot be circumscribed by a simple length limit. For some designs, clinical data collected from large patient cohorts showed a survivorship comparable to traditional stems. In cases that must be revised, this often can be performed with a conventional primary stem, fulfi lling the promise to preserve bone for potential future revisions in younger patients.

The authors are from the Orthopaedic and Traumatologic Department (FF, FC), Santo Spirito in Sassia Hospital, Rome; and the Orthopaedic and Traumatologic Department (MP), Nuova ITOR, Rome, Italy.

Dr. Papalia has no relevant fi nancial relationships to disclose. Dr. Falez is a paid consultant for Smith & Nephew, Lima, Samo, and DePuy; receives payment for lectures from Smith & Nephew, Lima, and DePuy; receives royalties from Smith & Nephew and receives payment for development of educational presentations from Smith & Nephew, Lima, and DePuy. Dr. Casella receives payment for lectures from Clinical Forum S.r.l.

Correspondence should be addressed to: Francesco Falez, MD, Orthopaedic and Traumatologic Department, Santo Spirito in Sassia Hospital, Lungo Tevere in Sassia n.1, 00193 Rome, Italy (fran-cesco.falez@francescofalez.it).

doi: 10.3928/01477447-20150215-50

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Few concepts in total hip arthro-plasty have led the academic debate or influenced surgical

practice more than tissue-sparing sur-gery. Most of this debate has regarded minimally invasive approaches during the past decade, although conservative femoral hip stems with conventional approaches have shown excellent re-sults, and in the early discussion, lim-ited clinical data on short stem designs were available.

However, new research and design evolution have spread the concept of bone preservation, and clinical studies have provided more data as a starting point for building a better scientific basis for new stem designs and tis-sue-sparing surgical procedures. The purpose of this study was to further develop and apply an existing compre-hensive classification system for short hip stem implants currently available.

CLASSIFICATION OF SHORT HIP CONCEPTS

There is still a lack of a proper clas-sification system for short hip stem implants, with a clear and accepted definition for implant characteristics and the extent of bone preservation in the metaphyseal and diaphyseal femur. Therefore, it is not always clear wheth-er a femoral hip stem can be classified as conventional or short, or even as something in between.

Recently, Feyen and Shimmin1 pro-posed a comprehensive classifi cation system for femoral components, based on the level of femoral resection and im-plant fi xation. However, in their categori-zation, implant type I (resurfacing), type II (mid-head resection), and type V stems with diaphyseal fi xation are intuitively distinguishable, whereas the distinction between type III (short stems) and type IV (traditional stems) was based on a length measurement that has not been validated. This length measurement was calculated by the authors as “total length

less than twice the tip of greater trochan-ter (GT) to base of lesser trochanter (LT) vertical distance.” Moreover, the pro-posed type III subclassifi cations for short hip stems in a solely subcapital III-A and standard III-B osteotomy may not refl ect all of the differences of femoral oste-otomy for currently available short hip stems. In any case, it must be considered that the primary fi xation principles and osteotomy level are intimately related and subsequently infl uence the geometry of the proximal short stem portion and the presence and eventually extension of its diaphyseal design toward the overall length of the femoral component.

Whether considering a brief distinc-tion between subcapital and standard osteotomy as described by Feyen and Shimmin1 or a classification of collum, partial collum, and trochanter-sparing stem designs as used by Van Oldenrijk et al2 in their comprehensive review, a definition of neck resection is an unde-niable characterization for short stems to understand their functional and bio-mechanical behavior to support a proper clinical comparison. In addition, some recent designs, clearly or less clearly derived from conventional straight stems, are basically a short-stemmed component with a limited but still ex-isting trochanteric violation, at least at the entry point of the fossa trochan-terica. Therefore, the current authors introduced a fourth category of tro-chanter-harming short stems, described as interruption of the circumferential integrity of the femoral neck section. With this type of implant, impaction of cancellous bone inside an intact corti-cal ring is no longer needed because of a “filling” fit in the metaphysis, with a larger proximal geometry (compared to trochanter-sparing designs) requiring the use of a box osteotome as the initial step of femoral preparation (violating the fossa trochanterica) (Figure 1).

Extramedullary anchorage (ie, thrust-plate hip system) has been considered

by Van Oldenrijk et al.2 This is a com-pletely different concept and will not be discussed in this article.

For all of these considerations, a clear definition of short hip stems is far from being accomplished and is not easy to define. Even in cases in which hip stems have been shortened as an ex-plicit evolution or range extension of a previous conventional component (eg, Taperloc Microplasty [Biomet, War-saw, Indiana] or CLS Brevius [Zim-mer, Warsaw, Indiana]), with the stem length varying in a way that the stems are borderline between short stems and conventional stems—if a given length will be validated—the definition is somewhat floating.

Restricting this overview to short stem implants (Table 1 and Figure 2), to components with a literature-based clinical outcome (phase 2 to 4 studies), and Level I to III longitudinal studies (ie, randomized controlled, controlled, and uncontrolled) or cross-sectional studies and case studies, van Oldenrijk et al2 defined at least 16 different femo-ral short hip stem designs.

However, some of these designs have been taken off the market, mainly for commercial reasons or for further evo-lution of the stems; these include the Biodynamic hip implant (Howmedica,

Figure 1: Osteotomy levels of the 4 short stem categories.

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Mahwah, New Jersey) that was es-sentially a precursor of the later CFP stem (Waldemar Link, Hamburg, Ger-many), Mayo stem (Zimmer), Delphi-M stem (ESKA Implants AG, Lubeck, Germany), and Proxima and Silent hip stems (DePuy Orthopaedics, Warsaw, Indiana). The Profi le stem (DePuy Or-thopaedics) and Citation stem (Stryker, Mahwah, New Jersey) seem quite far from being defi ned as short stem and have been excluded from the current study. The SMF short stem (Smith &

Nephew, London, United Kingdom), LPI Prime (Exactech, Gainesville, Flor-ida), GTS (Biomet, Warsaw, Indiana), and CLS Brevius stem (Zimmer) are included. The Trilock BPS bone pres-ervation stem (DePuy Orthopaedics), with a somewhat different conservative concept that focuses more on the nar-row cross-section than on the short stem character as stated by the manufacturer, was not included.

Therefore, for this study, a total of 15 contemporary short femoral designs

were eligible for inclusion; for 10 of these designs, published clinical results or experience reports are available. For the remaining 5 designs, data in inter-national clinical studies are expected in the near future. These implants can be divided into the categories described above, related to neck preservation and bone sparing (Table 1).

SHORT STEM RESULTSAs mentioned, clinical reports are

available for most of the short stems with reference to the system overview (Table 1); however, these studies show an extreme variability concerning their level of significance. Data for the Mayo stem and the CFP stem first ap-peared in clinical reports in the 1980s and 1990s3-6 introducing the concepts of short-stemmed femoral components.

Because of the marked differences in sample dimensions and duration of follow-up among all of the available clinical results, a corrective factor was used, based on cases revised per 100 observed component years, pointing out a benchmark of 90% of survivor-ship at 10-years of follow-up (or revi-sion rate <1 per 100 component years),2 consistent with the National Institute of Clinical Excellence (NICE)7 and pro-posed by the Quality of Literature in Arthroplasty project (QoLA) supported by the European Federation of Nation-al Associations of Orthopaedics and Traumatology (Effort) and European Arthroplasty Registry (EAR).8,9

Such unification of results reported for collum prostheses2 has shown a high revision rate of 2 per 100 observed

Table 1

Overview and Classificationa of Commercially Available Short Hip Stems

Level of Conservation Short Stem Stem Lengthb,mm Classifi cation

Collum Spiron 76-96 III A

Partial collum with neck preserving osteotomy

CFP 130-155 III A

Collo-Mis 99-123 III A

Nanos 97-128 III A

Mini-Hip 90-130 III A

Metha 98-123 III A

Optimys 95-139 III A

LPI Prime 102-107 III A

Trochanter sparing Taperloc Micro-plasty

112-140 III B

GTS 108-159 III B

Fitmore B 113-143 III B

Aida 107-155 III B

Trochanter harming CLS Brevius 123-172 III B

Profemur Preserve 97-125 III B

SMF 120-133 III B

aClassifi cation system developed by Feyen and Shimmin.1bMeasurement from medium head center to stem tip.

Figure 2: Short hip stems in the study (from left to right): Spiron, CFP, Collo-Mis, MiniHip, Nanos, Metha, Prime, Aida, Profemur Preserve, Optimys, Taperloc-Microplasty, GTS, Fitmore, SMF, and CLS Brevius.

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component years and an estimated sur-vivorship at 10 years of more than 90%, except for the Gothenburg Osseointe-grated Titanium (GOT) hip (Sweden),10 but this was only on the basis of one study with 40 patients observed after 24 months of follow-up. The well-docu-mented CUT stem (Eska, Luebeck, Ger-many)11-14 showed a revision rate close to 1% per 100 observed component years,11,14 but the overall rate reported in the literature for this particular stem missed the expected benchmark. In ad-dition, most of the components included in this group are no longer available for implantation (CUT, GOT) or are close to be taken off the market (Silent). The Spiron collum-type hip (K-Implant, Garbsen, Germany) is available for im-plantation, and a recent clinical study15 with short-term follow-up (1-3 years) reported 1 revision in 28 implanted stems.

The available clinical results for partial collum stems represent a differ-ent situation, appearing with an over-all rate of 0.64 cases revised per 100 observed component years2 that defi-nitely meet the NICE criteria. Exclud-ing systems without clinical reports or only short-term, biomechanical, or de-scriptive studies, Table 2 summarizes study results of currently commercially available partial collum and trochanter-sparing short hip implants published through mid-2014.

The longest mean follow-up (ap-proximately 10 years or longer) has been reported for the CFP implant.16,17 Mid-term clinical surveillance (>6 years) is available for the CFP,18-21 Mayo,22-25 Nanos26 (Smith & Nephew), and Metha27,28 (Aesculap AG, Tuttlin-gen, Germany) (implants. Short-term studies have been published for the Taperloc Micro,29 Fitmore30 (Zimmer), and GTS stems.31,32

The revisions per 100 component years (ie, time incidence rate) has been calculated for each study; cup-related

short hip stem revisions were excluded, as they should not influence the com-parison of survival performance of the femoral hip stem. For the Mayo hip,22 9 femoral stem revisions induced by polyethylene osteolysis were excluded. The described modular neck-related complications pertaining to the ini-tial titanium-alloy neck series of the Metha27 short stem were considered separately as well.

Published short-term results were only available in German in an over-view monograph about short hip stems by Jerosch33 and for the MiniHip34 (Corin, Cirencester, United Kingdom), Optimys (Mathys, Bettlach, Switzer-land),35 Collo-Mis36 (Lima, Udine, Italy), and Aida (Implantcast, Bux-tehude, Germany).37 Because no ad-ditional international literature was found, the review by Oldenrijk et al2 also was considered. The only trochan-ter-sparing stem with a revision rate per 100 component years >1 was the Aida short stem, with a rate of 2.3. The reason is exactly the same that can be described for Optimys (among partial collum stems), where a single revision in a small series with short follow-up may influence the calculation of the ex-pected mean revision rate per 100 com-ponent years. This seems reasonable, considering that the Aida short stem appears to be an evolution of the Mayo stem with improved surface finishing and that the Aida stem may reach the Mayo revision benchmark.

DISCUSSION OF CLAIMS VERSUS EVIDENCECollum Stems

From the literature review, it is imme-diately evident that the most conserva-tive group of short stems (collum type) have yielded the worst clinical results, with higher actual or estimated revision rates. More than one consideration sub-sequently arises about this group of con-servative femoral components. The fi rst

consideration is whether mechanical be-havior is a factor. Despite the fact that preliminary studies for one of these de-signs reported a satisfying reproduction of joint biomechanical parameters,11 a mid-term follow-up study13 described a tendency to valgization with femoral lateral offset decrease, periprosthetic ra-diolucency as well as calcar atrophy ex-pression of loosening, and stress shield-ing, with an unacceptable failure rate of nearly 50% at 8 years.

Other reasons for variation in clini-cal outcome may be that the implan-tation technique is technically de-manding or prone to a higher risk of intraoperative fracture or suboptimal positioning (with potential detriment to future stability and integration of the component, more than expected with short stems provided with larger metaphyseal anchorage). For the same reason, a potential limit in the appro-priate indications (beyond the simple age considerations) may be an explana-tion for failure, for example, if subopti-mal bone quality or abnormal anatomy of the femoral neck is ignored. Even if the use of these types of implants has decreased, further follow-up data may reveal valuable data on the biomechan-ics of the femoral neck. Further design developments may improve initial sta-bility, as a key factor in future stability. The authors are currently working on this by participating in the first clinical evaluations of the X-FIT component (Samo, Bologna, Italy).

Partial Collum and Trochanter-Sparing Stems

When looking at partial collum and trochanter-sparing short hip stems, it became evident that there was no dif-ference in these 2 types of stems re-garding survival rate. Both types of stems have a revision rate per 100 com-ponent years <1. As stated previously, the clinical survival of the Metha stem would remain consistently below that

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benchmark if neck breakages were ig-nored (9 cases at a mean follow-up of 4.9 years as reported by Wittenberg et al27). This particular failure mechanism has been related to a fatigue fracture of titanium alloy modular neck adapters, originating in surface cracks caused by fretting or fretting corrosion (appar-ently initiated by micromotion at the tapered connection).38 The titanium neck components were then replaced

by the manufacturer with cobalt-chro-mium adapters, reducing micromotion dramatically in mechanical tests39 with a subsequent decrease in fretting corro-sion and surface cracks.

Commercial reasons have entailed retirement of the Proxima femoral component that showed a definitely fa-vorable clinical behavior: the best revi-sion per 100 component years rate, lim-ited to the trochanter-sparing stem, as

reported by Van Oldenrijk et al2 in their review, at a short follow-up but a con-siderable sample size of 125 cases in 3 different studies.40-42 Future clinical surveillance of components implanted to date may reveal further information about biomechanics and implant-to-bone stimulation. Regarding the latter aspect, a supposed superiority of par-tial collum stems to load the proximal femur and thus avoid stress-shielding,

Table 2

Clinical Results of Short Hip Stems

Study Short Hip CasesFollow-up, y

Survival, %

Stems Reviseda

Time Incidence

Rateb Infection Complications

Morrey et al6 (2000)

Mayo 159 6.2 91 15 (4 early, 11 later)

0.61 0 Periprosthetic frac-ture (2), acetabular

loosening and osteolysis-related complications (9),

pain (1), mechanical loosening (3)

Falez et al25 (2008)

Mayo 160 4.7 98 4 (1 early, 3 later)

0.53 1 Periprosthetic frac-ture (1), mechanical

loosening (2)

Ettinger et al26 (2011)

Nanos 72 5.2 100 0 0 0 None

Grappiolo et al31 (2011)

GTS 570 1.5 100 0 0 0 None

Briem et al19 (2011)

CFP 155 6.2 99 1 (early) 0.1 1 None

Kress et al20 (2012)

CFP 84 7 97 1 (later) 0.17 1 None

Molli et al49 (2012)

Microplasty 269 2.4 99 1 (later) 0.15 1 None

Gustke30 (2012) Fitmore 500 1.3 99 3 (later) 0.46 1 Periprosthetic frac-ture (2)

Wittenberg et al27 (2013)

Metha 250 4.9 97 9 (7 early, 2 later)

0.65 3 Acetabular loosen-ing and osteolysis-related complica-tions (1), femoral

perforation (2)

Thorey et al28 (2013)

Metha 151 5.8 98 3 (early) 0.34 1 Mechanical loosen-ing (2)

Kendoff et al16 2013

CFP 149 11.2 96 5 0.30 1 Mechanical loosen-ing (4)

Hutt et al17 (2014) CFP 75 9.6 100 0 0 0 None

aEarly: <1 year, later: >1 year.bRevision rate per 100 observed components years.

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particularly in Gruen zone R1 com-pared to trochanter-sparing designs, seems to be contradicted by dual-energy x-ray absorptiometry (DEXA) findings reported in the literature.43,44 Besides a favorable stimulation of the calcar region (R7) in many conserva-tive stems belonging to these 2 groups, except for the Nanos stem, which showed a decrease in bone density in R7 in 2 reports,45,46 a slight-to-mild reduction in bone density was seen in the trochanteric region (R1) for most of the stems after follow-up of 1 year or longer, except for the Proxima conser-vative hip, in relationship to the lateral flare provided by this particular compo-nent as reported by Learmonth.47 More-over, for most of the designs, a stimula-tion in the distal portion of the stems (R3-R5) can be detected as a result of a partial distal loading potentially not researched by the stem designers.

Thus, the question remains whether stem choice is only justified by the need of bone stock preservation for future revisions. Theoretically, a par-tial collum device is replaceable with a trochanter-sparing stem, but preserving the trochanteric fossa during revision is difficult, and a trochanter-harming de-vice then appears to be more reliable in a possible conservative revision as well as a conventional standard stem.

Trochanter-Harming StemsResults of trochanter-harming femo-

ral stems are still lacking in the peer-reviewed literature, but results in line with standard or trochanter-sparing may be expected. The shortness of the component and the pure proximal pri-mary fixation should limit the proximal stress shielding observed with longer components, a statement to be proved with dedicated DEXA.

The key point in defi ning this fourth category of short stems, compared to trochanter-sparing components, is the in-terruption of the circumferential integrity

of the femoral neck section (at any given osteotomy height, required by the surgi-cal technique): impaction of cancellous bone inside an intact cortical ring is no longer needed because of a different fi t in the metaphyseal region. This different fi t is generally achieved with a larger proxi-mal geometry (compared to trochanter-sparing designs) and requires the use of a box osteotome as the initial step in femo-ral preparation (violating the trochanteric fossa); it allows for removing and impact-ing a part of the trochanteric cancellous bone with rasps and impactors to achieve a proper settlement of the component.

It does not mean harm to the trochan-teric structure, but rather the term tro-chanter harming states that the integrity of the greater trochanter, starting from the opening of the trochanteric fossa and moving laterally, is not a require-ment (or a consequence) of surgical technique, in opposition to trochanter-sparing components. A much-discussed topic is that many of the conservative stems have been designed with a neck modularity to improve joint biomechan-ical reconstruction. However, since the discovery of potential ion-related com-plications caused by neck-stem taper junctions, and the fact that a proper off-set usually can be defi ned preoperative-ly, the trend is moving toward the use of monolithic versions of the prosthesis. A particular situation may be considered if computer-assisted surgery is routinely used: the use of this technology allows the surgeon to select not only the cer-vicodiaphyseal angle but also ante- and retroversion intraoperatively to optimize range of motion and stability on the ba-sis of measurable and reliable data.48

Short stems are attractive for mini-mally invasive approaches, allowing an easier femoral preparation with short broaches. In a study by Molli et al49 comparing Taperloc standard and mi-cro stems, implanted via a minimally invasive approach, the intraoperative complication rate was significantly

lower in 269 hips receiving the short version of the stem.

CONCLUSIONThe complexity of this topic is not

easily decoded with a simple definition of a length limit, to which a definitive value is attributed for traditional and short lengths of a femoral component. If a given length will become a reliable limit for short stem definition, it may happen that components previously not qualified as “short” will be included in this family (eg, Trilock BPS), especial-ly in cases of a stem length comparable to one or more “shortened” hip stems.

Short stems, particularly when bone preservation has not been taken to the extreme of collum type implants, have proved to be reliable in terms of survi-vorship. Even if absorptiometry gener-ally detects less proximal bone stimu-lation than expected, particularly in the trochanteric region, most of these components can be revised with at least a conventional primary stem fulfilling one of the main goals: to preserve bone for future revision in young patients, with only few exceptions, such as in the case of periprosthetic fracture with an unstable component.

A suggestive field of application of short stems may be, in the near future, the development of cemented short de-signs. A shortened version of the Exeter stem (Stryker) is already under in vitro investigation,50 after the encouraging results achieved with a special series of Exeter stems implanted in Asia, shorter than conventional, with reduced offset and different taper angles to match the smaller femora in that specific popula-tion.51,52 It may seem that short stems and cemented stems are clashing terms, but perhaps it is possible that the short-ness of the component will reduce the complexity of cemented stem revision and that the long-term follow-up of ce-mented prostheses will increase poten-tial survivorship of short stems.

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■ Feature Article

42. Logroscino G, Ciriello V, D’Antonio E, et al. Bone integration of new stemless hip implants (Proxima vs Nanos): a DXA study: preliminary results. Int J Immu-nopathol Pharmacol. 2011; 24(suppl 2): 113-116.

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