Many different lengths of stem are available for use in primary total hip replacement, and the morphology of the proximal femur varies greatly. The more recently developed shortened stems provide a distribution of stress which closely mimics that of the native femur. Shortening the femoral component potentially comes at the cost of decreased initial stability. Clinical studies on the performance of shortened cemented and cementless stems are promising, although long-term follow-up studies are lacking. We provide an overview of the current literature on the anatomical features of the proximal femur and the biomechanical aspects and clinical outcomes associated with the length of the femoral component in primary hip replacement, and suggest a classification system for the length of femoral stems.
Cite this article: Bone Joint J 2014;96-B:442-8.
An important function of the femoral component in total hip replacement (THR) is to transmit the force generated at the centre of rotation to the proximal femur. In 1980, Crowninshield1 stated that many of the design variations in femoral components were the product of intuitive processes rather than the result of rigorous engineering analyses. There is still little scientific evidence to support the choice of length for most current designs of stem. Their length is often based on the average length of most components at the time of their design, rather than on an assessment of long-term performance. Roughly put, and to quote one of the designers of a current popular implant from personal communication in May 2013: ‘the stem had to be as long as necessary and as short as possible’.
When considering a primary THR, today’s orthopaedic surgeon is confronted with a wide variety of choices of stem. The design of both cemented and uncemented stems has undergone many modifications since Sir John Charnley successfully popularised the procedure. Despite the continuing debate about whether cemented or uncemented fixation is superior, recent developments tend to focus more on the preservation of bone.2 Short stems, which are designed to achieve a more anatomical pattern of stress distribution and the resection of less bone, are gaining popularity.3 From a clinical performance perspective these stems are designed to reduce the incidence of mid-thigh pain, which occasionally occurs with uncemented stems in active patients.4
The extent and type of coating, the geometry, surface finish and metallurgy of the stem are all important characteristics that will determine a specific stem’s identity and its behaviour in vivo. The focus of this review, however, is confined to investigating whether the length of the stem matters in primary THR.
Materials and Methods
We carried out a literature review on the evidence supporting the use of different stem lengths in femoral components currently in use for both, cemented and uncemented designs.
The Cochrane and PubMed databases were searched, using ‘femoral stem length’, ‘hip stem length’, ‘short stem femoral component’ and ‘anatomy proximal femur’ as search terms. All retrieved articles were assessed for suitability based on their title and abstract. Articles were deemed useful when reporting on the biomechanics of stem design; describing the proximal femoral anatomy with reference to the design of a stem; comparing stems of different lengths; or describing the biomechanical concepts and clinical results of shorter stems. In order to focus exclusively on the length of the stem in primary THR, articles were excluded that reported on the length of the stem in revision surgery, in hip resurfacing and in patients with dysplasia of the hip. This list was then further screened to look for additional suitable articles.
It became clear that there is no universally accepted classification of stem length, although a classification system for certain shortened stems has been described.5 For clarity of the objective of this review, we needed to provide a more extensive classification that can be used for all types of stem that are available for primary THR, based on their length (Fig. 1), the level of the osteotomy of the femoral neck (neck preserving versus standard) and the intended site of primary stability (metaphyseal, meta-/diaphyseal, diaphyseal). This allowed us to have a more organised discussion on the issue of stem length. We therefore classified stems into five different types, with a subclassification for types III and IV based on the level of resection of the femoral neck and the site of fixation, respectively (Table I).
|III||Short stem (total length less than twice tip of GT to base of LT vertical distance)|
|IIIA||With subcapital osteotomy|
|IIIB||With 'standard' osteotomy|
|IV||Standard stem (total length greater than twice distance from tip of GT to base of LT vertical distance)|
|IVA||With metaphyseal fixation only|
|IVB||With meta- and diaphyseal fixation|
GT, greater trochanter; LT, lesser trochanter.
The purpose of this review was to focus exclusively on type III and IV stems, and to investigate the effect of various lengths of stem on the anatomical fit and the biomechanical and clinical outcomes.
Proximal femoral anatomy
For optimum fixation in the proximal femur a thorough knowledge of the local anatomy is essential, especially in cementless fixation, where a close proximal fit optimises the initial torsional stability,6-8 thereby facilitating bony ingrowth9-11 and minimising fibrous ingrowth.11 Direct support of the stem by the strongest available bone, lying within 2 mm to 5 mm of the cortical wall, is only possible when implants closely approximate the endosteal geometry of the proximal femoral canal. The importance of metaphyseal fit in uncemented stems in order to achieve the physiological transfer of load from the stem to the bone and to minimise stress shielding and adverse bone remodelling has been demonstrated.12,13
Much of our knowledge of proximal femoral anatomy as a basis for the design of the femoral component was elucidated by Noble et al.14 They found a correlation between head size, femoral length, femoral offset and external cortical diameter, thus creating a relatively proportional silhouette of the femur. In contrast, they suggested that the diameter of the femoral medullary canal was not proportional to these characteristics. The canal flare index (CFI) (Fig. 2), defined as the ratio of the intracortical width of the femur at a point 20 mm proximal to the lesser trochanter midpoint and at the isthmus of the canal, shows wide variations, ranging from 2.4 to 7.0 in the 200 investigated specimens, indicating that the femoral medullary canal does not have a universally reproducible shape.
A CFI < 3.0 was classified as being a stovepipe canal, whereas CFIs ranging from 4.7 to 6.5 were classified as having a champagne-flute configuration.14 This concept was later modified and popularised as the Dorr classification (A, B and C).15 The poor correlation between the proximal and distal dimensions of the femoral canal necessitates stems being selected on the basis of their fit in the proximal rather than the distal canal, thereby optimising metaphyseal load transfer. Dorr et al15 also found that the mediolateral diameter of the femoral canal at a point 20 mm distal to the lesser trochanter has the most predictable relationship with external femoral dimensions and could be estimated with an accuracy of ±12% (95% confidence interval, CI). These findings provide an anatomical basis for the metaphyseal fixation of certain types of uncemented stem.
The wide variations in the dimensions of the femoral canal have been confirmed in CT-based studies,16,17 which also described poor correlations between the dimensions of the femoral canal 20 mm above and 20 mm below the level of the lesser trochanter. In both men and women, the medullary canal widens with age.14,18 The decrease in corticomedullary ratio with ageing is less pronounced in men, owing partly to the cortical expansion that occurs in men in addition to resorption of medullary bone.19 In addition, in women there is the hormonally based increase in the resorption of medullary bone in the menopausal decade.20
The presence of a femoral stem does not influence this expansion of the medullary canal with ageing, as described by Poss et al.21 Whether the expansion of the femoral canal that occurs with age contributes to loosening of the femoral component remains unknown.
Biomechanics of the proximal femur
In accordance with Wolff’s law,22 the presence of a stem in the proximal femur alters the distribution of load and subsequently influences the continuous process of bone remodelling. The insertion of a stem, either cemented or uncemented, inevitably leads to a reduction in compressive, tensile and shear strains in the proximal femur, regardless of the type of stem.23,24 When using an uncemented stem, normal patterns of strain are approached when a tight proximal fit of the stem is achieved,24,25 whereas a tight distal fit can significantly reduce proximal strains.24 The closer the contact of the distal part of the stem, the more proximal stress shielding occurs, whereas the absence of contact between the stem and the distal cortex may reduce stress shielding, bone resorption and thigh pain.24 Hence the length of the stem plays an important role in the transfer of forces to the femoral bone. Conceptually, reducing the length of the stem reduces proximal stress shielding, at the cost of a reduced contact area for fixation and load transfer.
In biomechanical studies, Bieger et al26 and Arno et al27 suggested that shortening a femoral stem reduces proximal stress shielding without compromising primary stability. They also concluded that a metaphyseal-only design biomechanically provided the best match for the native femur. Boyle et al28 compared the biomechanics of the Alloclassic (Alloclassic Hip System, Zimmer Orthopaedics, Warsaw, Indiana) with the short-stemmed Mayo component (Mayo Conservative Stem, Zimmer Orthopaedics, Warsaw, Indiana) and found a more effective load transfer to the cancellous bone with the Mayo stem, combined with decreased proximal bone loss. Østbyhaug et al29 investigated how much shortening of the ABG-1 stem (Stryker Howmedica Osteonics, Newbury, UK) was possible before stability was compromised. Reducing its length by 40 mm to 50 mm, thereby creating a stem that extended 30 mm to 40 mm below the level of the lesser trochanter, did not compromise stability but virtually normalised the load distribution in the lower metaphysis and upper diaphysis.30
Contrary findings were found by Van Rietbergen et al,31 who stated that the differences between the predicted bone resorption patterns were minor, and not always in favour of a shorter design. In addition, compared with the longer stem, higher shear stresses were observed near the distal end on the lateral side of the short stem. Furthermore, there was a paradoxical shift of the distribution of stress distally in a midsize metaphyseal-only stem compared with a more generalised distribution in the original design. In addition, they concluded that shortening the stem may be associated with a decrease in initial stability.
Ong et al32 compared a longer and a shorter version of the Omnifit hydroxyapatite, (Stryker Orthopaedics, Mahwah, New Jersey) stems and found that although there was more potential for bone formation in the medial calcar and a slight decrease in proximal bone resorption with the shorter design, this came at a cost of significantly reduced primary stability, with movement at the bone–implant interface being 40% to 94% greater than with the longer stem.
In the case of cemented stems, Crowninshield et al1 reported that an increase in length from 100 mm to 130 mm resulted in a 31% increase in the predicted maximum tensile stress in the stem, but a decrease of 26% in the predicted maximum compressive stress in the cement and hence in the bone, suggesting that increased length may not always be advantageous.
The rotational stability of the Exeter stem, which is based on a taper-slip engagement principle, has been biomechanically studied for varying stem lengths.33 Shortening the stem did not reduce its rotational stiffness, and the authors concluded that the proximal geometry of the stem is the aspect that contributes most to rotational stiffness when applying these fixation principles.
The risk of fracture
Bishop et al,34 in an analytical model, highlighted an increased risk of bone overload in shortened uncemented femoral stems, especially in the presence of poor bone quality. Increased bone stresses with decreasing length of stem were noted, and they concluded that adequate patient selection is essential when using short stems. Their findings were not, however, supported by Jakubowitz et al,35 who artificially reproduced peri-prosthetic fractures in femoral specimens when comparing cementless Spotorno (CLS stem; Zimmer Orthopaedics, Warsaw) with Mayo short stems, and found a slight but non-significant reduction in resistance to fracture with the short stem.
In shortened cemented Exeter stems (Stryker Orthopaedics, Mahwah, New Jersey) there is a significant reduction in load to fracture compared with stems of a conventional length when tested in a loading frame under combined compression forces and torque.36 However, the torque to failure still exceeds the torques observed in activities of daily living by a factor of 7 to 10, so the authors concluded that both designs can be used safely.
Clinical studies on short-stemmed implants
Clinical results on short uncemented stems are summarised in Table II. Although there is an abundance of papers describing the use of these short uncemented stems with short follow-up, there are few that report mid-term results (Table II) and none that report the long-term outcome. Morrey,37 in 1989 was the first to publish clinical results following the use of a short stem, with one revision because of early loosening in 20 Mayo stems (type IIIb) at one-year follow-up. In a study involving 159 Mayo stems with a mean follow-up of six years, the same group reported excellent initial stability and a 98.2% survival with revision for aseptic loosening as the endpoint38; however, in 11 hips (6%) there was evidence of proximal osteolysis and two patients needed revision because of osteolysis more than seven years post-operatively. It should be noted that, although not mentioned, conventional polyethylene liners were probably used in this study. A total of 12 stems (7%) subsided by > 2 mm. Furthermore, an intra-operative fracture occurred in ten hips (6%), all of which united uneventfully after cerclage wiring. A more recent study by Molli et al,39 in a large retrospective series comparing standard-length stems with a short stem (type IIIb design), found a decreased intra-operative rate of fracture with short cementless stems.
|Paper||Year||Stem||Class||Number||Age||Gender M/F||Follow- up (yrs)||Intra-op fracture (%)||Survival (%)||Revision (loosening, osteolysis, subsidence)||Subsidence Not revised|
|Morrey37||1989||Mayo||IIIb||20||Not recorded||3/17||1||1 (5)||95||1 (5)||Not recorded|
|Morrey, Adams and Kessler38||2000||Mayo||IIIb||159||51||74/72||6.2||10 (6)||98.2||3 (1.8)||20 (12)|
|Briem et al40||2011||Collum Femoris Preserving (CFP)||IIIa||155||59||80/75||6.2||0||99.4||1 (0.6)||Not recorded|
|Gustke3||2012||Fitmore||IIIb||500||67||210/290||1.3||1 (0.2)||100||0||34% (of first 100 cases)|
|Schmidutz et al41||2012||Metha||IIIa||82||55||43/31||2.7||Not recorded||100||0||6 (7.5)|
|Wittenberg et al43||2013||Metha||IIIa||204||60||86/118||4.9||2 (1)||97.5||5 (2.5)||8 (3.9)|
|Ettinge et al44||2011||Nanos||IIIa||72||63||32/33||5.2||0||100||0||0|
|Santori and Santori45||2010||Custom made||IIIa||131||51||60/49||8||7 (5.3)||100||0||0|
|Patel et al46||2013||Custom made||IIIb||69||56||38/31||5.4||0||100||0||0|
|Kim et al47||2011||Proxima||IIIa||84||79||38/46||4.6||1 (1.2)||100||0||0|
|Kim et al48||2013||Proxima||IIIa||256||65||129/101||5.6||Not recorded||99.6||1, (0.4)||2 (0.4)|
A survival rate of 99.4% was reported by Briem et al40 for the Collum Femoris-Preserving (CFP, Collum Femoris Preserving, Waldemar Link GmbH, Hamburg, Germany) (type IIIa) stem at a mean follow-up of 74 months, with only one revision in 155 THRs because of aseptic loosening at one year post-operatively. There were no intra-operative fractures. There was some proximal femoral osteopenia, and cortical sclerosis frequently occurred around the distal aspect of the stem. They concluded that proximal stress shielding could not always be avoided by the use of a short stem. Gustke3 described a series of 500 THRs using the Fitmore stem (Zimmer Orthopaedics, Warsaw, Indiana) (type IIIb) with a mean follow-up of 1.3 years. There was one intra-operative fracture of the calcar (0.2%), and at this short-term follow-up survival was 100% for revision for aseptic loosening and 99.4% for revision for any reason. However, in the first 100 THRs there was subsidence of ≥ 2 mm in 34 stems, none of which led to symptoms. The incidence of subsidence thereafter decreased, highlighting the learning curve for this design of short stem.
Schmidutz et al41 reported a mean subsidence of 0.7 mm at a mean follow-up of 2.7 years when using the Metha stem (B. Braun Aesculap, Tuttlingen, Germany) (type IIIa). This is similar to the rate of subsidence seen with traditional type IV stems.42 In another study involving 250 Metha stems, five (2%) had to be revised in the five-year follow-up period for non-infective stem-related complications.43 There have been two reported studies with a 100% survival rate for type IIIa stems, at a mean of 5.2 years for the NANOS stem (Smith & Nephew, Marl, Gemany) and eight years for custom-made stems (Stanmore Implant Worldwide, Stanmore, UK and DePuy International, Leeds, United Kingdom).44,45 Similarly, a 100% survival rate at a mean of 5.4 years’ follow-up has been reported for a custom-made type IIIb stem (Biomet, Warsaw, Indiana).46 In one of these studies a rate of intra-operative fracture of 5.4% was reported; all were treated by cerclage wiring without subsequent subsidence.45 Thigh pain was not reported in any of the studies involving type III stems.
Most of these studies were performed in relatively younger patients with adequate bone stock. After previously reporting stable bony ingrowth when using the Proxima stem (DePuy International, Leeds, United Kingdom), which is a metaphyseal type IIIa stem, in patients aged > 70 years,47 Kim et al48 reported its use in patients with osteoporosis. At a mean follow-up of 5.6 years, a similar survival rate was recorded in 81 THRs with Dorr A bone, 83 with Dorr B bone and 92 with Dorr C bone. The authors concluded that poor bone quality was not a contraindication for the use of this stem.
Sluimer et al49 found an increased incidence of thigh pain in patients in whom a shorter type IIIb stem had been used compared to those with longer type IV stems between six and 24 months post-operatively, after which it settled. It was suggested that this might be linked to inferior primary stability with the shorter design, and to the position of the tip of the stem adjacent to the less stiff proximal bone compared to the stiffer distal bone in longer-stemmed designs.33
In a study comparing a conventional type IV stem with the type IIIa Metha stem, the short stem was deemed inferior in terms of restoring offset and leg length.50 The authors attributed this to the higher level of osteotomy of the femoral neck in this neck-preserving design. They also noted a wider range of varus/valgus positions with the shorter stem, confirming findings previously reported by Kamada et al.51 Shortened femoral stems have been reported to be particularly useful when using a direct anterior approach to the hip joint, allowing them to be inserted in a curved manner.52
For cemented stems, the initial Exeter design failed to reproduce their excellent results in smaller Asian patients.53 In this population, a higher percentage of high canal flare indices, and hence of champagne-fluted endosteal morphology, has been described.54 As a consequence there is a higher incidence of oversizing the stem at operation, with subsequent failure rates of 22% at a mean of 13 years post-operatively.53 Thus, shorter stems with different taper angles and with reduced offsets have been designed to allow for adequate restoration of offset and the recommended minimum cement mantle thickness of 2 mm.55 These stems are 95 mm, 115 mm and 125 mm long, with respective offsets of 30 mm, 33 mm and 35.5 mm. In short, mid- and long-term follow-up studies their results are comparable with those of longer stems.56-60 In a study involving 47 THRs, Tai et al59 reported a survival rate for the stem of 100% at a mean follow-up of 8.5 years. Choy et al57 reviewed the Australian Joint Registry and noted a femoral revision rate for any reason of 1.1% in both short and standard Exeter stems at seven years’ follow-up. Williams et al60 reported a 100% survival rate in 192 primary THRs for the short 35.5 mm offset stem at a mean of eight years, with aseptic loosening as an endpoint. Survival rates of 100% and 89% at respectively 10 and 12 years have been reported by Chiu et al56 in 45 THRs. The theoretical concern of fracture in these shortened stems has been raised,59 but to our knowledge, only two fractures of these shorter stems have been reported so far.60,61
The shortened cemented Exeter-type stems seem to perform as well as the original design of this stem.56-60 However, the short stem designs with decreased offset in these studies are typically used in Asian patients with a smaller stature and femoral size, thus still being proportionate to the femur in which they are implanted.56,58,59 They are not to be confused with the shortened conventional Exeter stems, the biomechanical aspects of which were investigated by Morishima et al.36 The length of these stems was reduced from 150 mm to 125 mm, leaving other aspects – including the offset – unchanged. These shortened, standard Exeter stems are soon to become commercially available, and clinical data are awaited.
The length of contemporary stems has been based on intuition and historical developments, rather than scientific evidence. We have suggested a methodology of classification.
Shortening femoral components has the biomechanical advantage of loading the proximal femur in a manner that mimics physiological loading. It may therefore reduce proximal stress shielding, thereby potentially reducing the long-term risk of peri-prosthetic fracture. In cementless stems these advantages come at the cost of a possible compromise to initial stability, as well as a possible increased risk of early peri-prosthetic fracture in these patients, especially those with osteoporosis. This compromised initial stability is not an issue in the collarless polished tapered type of cemented stems, since stability is provided by the geometry of the proximal part of the stem within the cement mantle.
In retrospective studies, short- and mid-term survival analyses show promising results for the shorter, uncemented bone-preserving designs3,37-44 (Table II). However, there is a learning curve associated with their use, and the surgeon should appreciate the requirement for achieving initial stability and avoiding implantation in the varus position. As discussed earlier, there seems to be less thigh pain than with standard stems. Restoration of limb length and offset appears to be more difficult with type IIIa stems owing to the higher level of osteotomy of the neck, leaving the surgeon with less control of these parameters.
It would seem reasonable that, given the excellent results of contemporary stem designs, any wholesale change in length would need more extensive clinical evaluation. Registries would seem to be the obvious source of data, but it will be difficult to determine any practical clinical benefit if revision is used as the only outcome measure. For example registries currently will not capture any reduction in the incidence of activity related thigh pain. Randomised trials recording the incidence of thigh pain and objectively measuring the bone mineral density of the proximal femur with the shorter stems are required. These studies should also look at differences that may occur with variations in proximal femoral anatomy (Dorr A, B, C). The difficulties of performing randomised clinical trials in this field are, however, well known.
Surgeons should always consider the potential need for revision, as infection cannot be totally eradicated as a clinical problem. In addition, an increasing number of younger patients with longer life expectancy are undergoing THR. Whether or not a reduction in the length of the stem will lead to easier revision and a better outcome after revision will take some time to evaluate.
In conclusion, when choosing a stem, surgeons have several objectives. They should aim for optimal distribution of stress in the proximal femur; for maximum preservation of bone without compromising stability; and for long-lasting fixation. Depending on patient-specific factors such as bone mineral density, proximal femoral anatomy and cortical–medullary ratios, this may require different types of stem design with different mechanisms of fixation, and different lengths of stem in different patients.
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No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article.
This article was primary edited by J. Scott and first proof edited by D. Rowley.