Abstract
Introduction
Total hip replacement is a successful procedure with long survival records compared to other joint arthroplasties. Cemented implants have been available for many years, however the complications associated with loosening and, ultimately, failure over time has led to the development of cementless stems and implants.
The ideal prosthesis should recreate a biomechanically normal hip joint, allow pain-free function and last the patient's life span without requiring revision. Optimal results with uncemented femoral stems rely on obtaining initial stability, osseointegration, biological fixation, and uniform stress transfer to the proximal bone.
There are a multitude of factors that can affect the integration, stability and fixation of these stems into bone, and understanding these factors is the key to choosing the appropriate implant for a specific femur.
Methods
This article aims to discuss cementless prostheses based on evidence-based practice. Geometry, roughness, stem coating, technique and bone quality are among the factors discussed. This was achieved through a review of the current literature.
Conclusions
Uncemented femoral stems have shown good, long-term survivorship and functional outcome, with promising results in younger patients.
Limitations in the current literature make it difficult to assess and compare different designs to determine optimal indications for each type.
Biological fixation, in which the prosthesis is directly fixed to the bone, is the preferred fixation method.
Future studies of cementless implants should consistently address patient age, activity level, bone type, and deformities so that more definitive conclusions can be drawn about when to use each design.
Introduction
Following the introduction of the total hip replacement (THR) in 1959 (1), various challenges have arisen in the pursuit of a more durable prosthesis.
Problems related to the fixation of femoral components with acrylic cement began to emerge with the appearance of osteolysis secondary to inflammatory reactions against polyethylene (PE) and cement particles (2, 3). As a result, considerable laboratory and clinical investigations have been carried out in an effort to provide for biological fixation of cementless femoral components in the search for a more durable prosthesis.
The first cementless designs, characterised by straight and very stiff distally fixed cementless stems, had significant stress-shielding phenomena (4, 5). To avoid these problems, new fixation systems were developed (6) and new models designed with the aims of enhancing initial press-fit stability, reduce stress-shielding and proximal bone loss, and improve anatomic orientation of the hip joint.
Despite the development of new cementless implants, osteolysis remains (7). As PE wear is the first step in osteolysis, it is still the most serious threat to implant survival (8). Current cementless hip replacements aim to completely seal the proximal femoral canal to reduce particle-related osteolysis (9, 10) whilst being compatible with non-polyethylene bearing surfaces.
Primary stability, as in conventional THR, is the key to good clinical and radiological results (11). A number of factors influence the initial stability of both conventional and short femoral stems: shape, surface texture, surgical technique and bone quality. In the ideal situation these factors provide primary fixation for the first 4 to 12 weeks, minimising micro-motion and thus promoting bone ingrowth or ongrowth (12).
The aim of this article is to discuss a number of factors that influence the initial stability or primary fixation of cementless stems. These include roughness and stem coating, material and the implant geometry.
Methods
A thorough electronic healthcare database search was conducted including:
Discussion
Cementless stems can be discussed based on their properties, a systematic approach in discussing factors affecting the stems has been undertaken.
Surface coating
The surface coating and roughness have important implications when it comes to initial stability and reducing osteolysis. Initial stability is required to allow for osseointegration, which has been described as the attachment of lamellar bone to implants without intervening fibrous tissue (13). This process takes approximately 4 to 12 weeks after implantation and may continue for up to 3 years (14, 15).
Firm fixation of the implant provides adequate osseous contact and reduces micromotion, enhancing bone ingrowth. The 2 prerequisites for bone ingrowth are immediate mechanical stability at the time of surgery and intimate contact between the porous surface and viable host bone. To fulfil these requirements, implants must be designed to fit the endosteal cavity of the proximal femur as closely as possible. Initial fixation is obtained by press-fitting a slightly oversized component. Ingrowth occurs when bone grows inside a porous surface. Ongrowth occurs when bone grows onto a roughened surface. The surface characteristics of an implant determine how the bone reacts to it, and will be discussed in detail.
Micromotion between the implant and host bone, pore size and the size of the gap between the bone and the implant all influence bone ingrowth. Excessive motion of approximately 150 μm or more between the implant and host bone leads to fibrous tissue formation rather than bone ingrowth (16, 17). Motion between 40 and 150 μm leads to a combination of bone and fibrous tissue formation and motion of <20 μm results in predominantly bone formation (18).
One of the most important clinical problems with cemented and cementless femoral design has been osteolysis resulting in periprosthetic bone loss. This process can lead to aseptic loosening and failure. Osteolysis is mediated by macrophage activated wear debris (predominantly due to the PE) arising from the joint bearing surface. Early cementless designs utilised porous coating applied in patches around the stem, allowing migration of PE debris in the joint fluid along the stem and to the distal femur through channels in the areas of porous coating. Circumferential coating therefore provides a seal, which minimises migration of wear particles and prevents distal osteolysis (19, 20).
Cementless femoral stems now have a circumferential and continuous surface coating (19, 21, 22). These qualities enhance metaphyseal osseointegration and proximal stress transfer and decrease bone loss from stress-shielding (23). Stems without a circumferential coating have been found to have high failure and osteolysis rates (19, 24, 25).
Porous coating
Most of the technology related to porous implants in hip stems have concentrated on the production of metallic devices with porous surfaces. They support tissue ingrowth and ongrowth (25) which promotes fixation to the femur and increases the longevity of the implant. This surface can be manufactured by sintering, diffusion bonded titanium, and titanium plasma spraying.
Sintering is a high-temperature process that allows particle-to-particle and particle-to-substrate bonding at contact points, generating a sintered bead surface (26). Titanium alloy and cobalt chromium porous surfaces can be produced. The sintering process lowers the fatigue strength of an implant made of either material by 20%-40% (27).
Fibre mesh coatings are produced by diffusion bonding (26). Compared to sintering, this is a relatively low-temperature process, and is used in combination with high pressure. Fibre metal titanium porous surfaces are created using this process. The notch sensitivity effect that is produced as a result of this technique decreases the fatigue strength of the implant by redistributing the carbide phase in the cases of chromium alloys and by creating notches at the sites of attachment in these cases (27).
Ongrowth surfaces are created by grit blasting or plasma spraying. Plasma spraying involves mixing metal powders with an inert gas that is pressurised and ionized, forming a high-energy flame. The molten material is sprayed onto the implant creating a textured surface. There is less interconnecting porosity than with the ingrowth surfaces; however, 90% of the implant fatigue strength is retained, whereas only 50% is retained after diffusion bonding and sintering (27, 28).
Grit blasting creates a textured surface by bombarding the implant with small abrasive particles such as aluminium oxide (corundum). The surface roughness ranges from 3 to 5 μm (29). Grit blasting can be used to enhance osseointegration with titanium implants, and as such, many manufacturers use a combination of proximal porous coating and distal grit-blasting on their stems (30).
Extensive porous coating of femoral stems has been implicated as a cause of adverse femoral bone remodelling. Unfortunately, there is significant difficulty in extracting a well-ingrown device with extensive porous coating without causing severe damage to the remaining stress-shielded femoral bone stock. Extensive coatings are often reserved for revision arthroplasties in which distal fixation must be relied on for implant stability.
Bone ingrowth after surgery occurs through a series of events that are similar to fracture healing; inflammation, repair and bone remodelling (14). During the first days after surgery, coagulated blood fills voids between the implant and bone. Up to a few weeks after surgery, the haematoma is invaded by mesenchymal cells, promoting osteoblast formation. The optimal pore size is in the range of 100-400 μm, corresponding to the pore size of trabecular bone (300 μm) (31).
Porous metals have a uniform 3-D network (31) with high interconnectivity of the voids and a high porosity (75% to 85%) compared with that of sintered beads and fibre metal coatings (30% to 50%).
Hydroxyapatite coating
Two thirds of the dry weight of bone is inorganic mineral hydroxyapatite (Ca10(PO4)6(OH)2) (32). Hydroxyapatite (HA) applied to a metal substrate or over a porous surface by plasma spraying or chemical deposition has been shown in experimental and in vivo settings to be osteoconductive (33–35).
HA has the ability to enhance mineralised bone growth across a gap, increase the attachment strength of implants, and increase the interface shear strength (36, 37). Clinical retrieval studies have shown bony ingrowth onto the HA coating from 10% to 20% of the surface after 3 weeks, 48% after 12 weeks and 32% to 78% after 5 to 25 months (35).
HA has also been shown to be capable of converting a motion-induced fibrous membrane into a bony anchorage (35) and facilitate rapid development of osseointegration (38). It also allows the surgical technique to be more forgiving by bridging gaps of up to 2 mm and mitigating the adverse effects of initial micromotion (34, 39).
Experimental data have shown the optimal thickness of the coating to be 50 μm (33). Thicker coatings can lead to fatigue failure under repeated cyclical loading and under sufficient stresses, compromising its strength (33, 37).
An important concern with HA coating is degradation that may lead to implant loosening (35, 40). HA resorption is necessary to trigger its basic osteoconductive effects, but in cases with micromotion, resorption can be accelerated leading to loss of bonding strength, fixation and delamination (41).
The porosity of currently available HA coating varies from 5% to 20% (35). A porous surface underneath the coating has been shown to be better than a grit blasted surface (42), although there has been some concern about interface strength and abrasion due to the release of particles with HA-coated implants, and resultant loss of mechanical fixation (43, 44).
Material
In the design of uncemented femoral prostheses it is important to take into consideration the stress transfer from the prosthesis to the bone while maintaining the fatigue strength of the component. In achieving these goals, experience has been confined largely to the use of cobalt-chromium and titanium alloys. Both materials have proved to be satisfactory so far.
Cobalt-chrome molybdenu
In the early 1970s, Cobalt-chromium-molybdenum (CoCrMb) alloy with a porous or sintered beaded surface (14, 45) allowing ingrowth of living bone were introduced.
The arguable advantages of cobalt chromium include its material hardness, with a potential decrease in the wear debris. Additionally, due to less notch sensitivity, it more readily maintains strength during the application of beads.
Titanium
Titanium alloys exhibit a superior biocompatibility, higher fatigue strength, and a lower modulus of elasticity (femur modulus of 12 GPa, titanium modulus of 110 GPa, and cobalt-chromium modulus of 220 GPa) (13, 46–48). If titanium is alloyed with 6% aluminium and 4% vanadium (Ti-6AI-4V) it was found to have superior strength while still having a favourable modulus of elasticity when compared to CoCrMb. Titanium alloys have the ability of obtaining more ingrowth, creating direct structural and functional connections between bone and the surface of the implants (13, 46).
At present, Ti-6AI-4V is the alloy of choice for most uncemented femoral stems. However, the use of this alloy as a bearing surface can lead to disastrous failures as a consequence to wear debris from metallic heads leading to accelerated osteolysis and loosening (49). Modular heads made of CoCrMb heads, oxidised metallic alloys/zirconium such as Oxinium™ (Smith & Nephew) (50) or ceramic materials are therefore used in combination with these stems.
Stiffness
The principle of cementless stem fixation requires the prosthesis to be of sizeable diameter in order to fill the canal, achieve cortical contact and stability. The stiffness of an implant is the ability of the prosthesis to resist bending and is calculated by the product of the materials modulus of elasticity and the moment of inertia. The modulus of elasticity is a property of the material determined by the choice of implant. The moment of inertia is based on the cross-sectional shape and size of the implant, and is proportional to the 4th power of the diameter.
The mismatch in the stiffness of the implant and the femur carries concerns for the potential failure of the prosthesis in several aspects.
Firstly, the stem bears the majority of the load, and prevents natural loading of the femur, where the pattern is reversed following implantation of the stem, leading to proximal unloading. The majority of the compressive loads are transferred to the metadiaphyseal region (51). Areas of the femur subjected to lower interface stresses become susceptible to bone resorption over time (calcar atrophy) (52), and there is distal diaphyseal bone formation (bone hypertrophy). This “adaptive bone response” is more apparent around bigger and stiffer femoral stems (12, 53, 54).
Proximal bone resorption is commonly seen after insertion of an uncemented stem. The average decrease in bone mineral density (BMD) has been reported to be between 4% and 45% (55–57). When bone loss is excessive it may be responsible for stem migration, aseptic loosening and fractures, this can pose technical problems during revision surgery. The occurrence of stress shielding is largely unpredictable but a correlation with implant material and size has been proven (6, 52, 58).
A second major problem of some traditional femoral implants is thigh pain. Almost all traditional long implants with any stem geometry are, to some extent, associated with thigh pain and stress shielding (11, 59). The cause of thigh pain is multifactorial; micromotion, excessive stress transfer, stem material and design, and bone morphology contribute to its aetiology (59).
Tapered distal stem geometries are inherently less stiff than cylindrical ones and have been associated with minimal thigh pain. Titanium alloy has been recommended as the material of choice in the past because its modulus of elasticity is approximately half that of CoCr alloy.
The role of different implant materials with different elastic moduli were held at least partially responsible for thigh pain post-operatively (60, 61). Comparison of implants with similar designs of different alloys showed no significant difference in the outcomes or rates of thigh pain (60, 62). Lavernia et al (60) studied titanium and CoCr alloy stems of an identical tapered design in 241 patients. Thigh pain was found to be unrelated to the material composition of the stem, but was more common in patients with a larger stem size.
The addition of deep, longitudinal grooves reduces bending and torsional stiffness. The bending stiffness in the distal third of the stem can also be reduced substantially by splitting the stem in the coronal plane, similar to a clothespin (Fig. 1).
A different and somehow extreme attempt to avoid thigh pain and prevent bone resorption due to stress shielding is the model of isoelastic stems. These implants of composite design are aimed at reproducing the natural flexibility of the human femur (63). Unfortunately, most flexible-isoelectric stems create high proximal stem-bone interface stress, causing interface debonding, relative motion, and femoral component loosening (64).
Different short implants with varied design philosophies inspire an apparently different bone response. Briem (65) reported a pattern of remodelling with the appearance of cortical thickening in the distal part of the column femoris-preserving (CFP) stem, thus confirming negative proximal bone remodelling with this device. Lazarinins et al (66) found substantial loss of periprosthetic bone density following the use of the CFP stem, with forces transmitted distally.
To attempt reduction of stress shielding and thigh pain, as well as to improve survivorship of the implant, the Mini THR and short stems offer a theoretical solution. Obtaining an optimal fit of the cementless stem in the proximal metaphysis of the femur negates the requirement for distal stem fixation, attempts at preserving the femoral canal, and femoral elasticity.
Shape
Femoral stems today, are referred to as proximally coated tapers or fully coated cylindrical, anatomical or straight, or by a system based on the amount of osseous contact and the progression of stem fixation from proximal to distal (11).
Dorr et al (67) developed a classification system that will guide the choice of implant selection, based on the canal/calcar isthmus ratio. The implant shape determines cortical contact and initial stability, and porous coating is located where fixation is desirable. In broad terms, fixation can be considered proximal metaphyseal or distal. The aim ultimately is to obtain initial stability, bone contact and osseointegration. Most straight or anatomical stems achieve stability thanks to a tight mechanical fit in the lower metaphyseal region or in the proximal third of the femoral canal. Excellent long-term results have been seen with stems that are HA coated and achieve both proximal and distal fixation as seen with the Corail stem (DePuy).
The geometrical classification of cementless stems is challenging, as there are numerous implants that cannot easily be classified into any 1 category, but utilise a combination of implant philosophies to obtain fit, stability and fill in the femur.
Fully coated cylindrical stems (distal fixation)
1 example of an early-uncemented implant is the anatomic medullary locking (AML) stem (Depuy, Warsaw, Indiana). These stems were cylindrical, with extensive porous coating along the whole length of the implant (6, 52). These stems require distal reaming and proximal broaching, and require endosteal bone engagement for cortical bone ingrowth.
The AML stems and some of the earlier fully coated stems achieved good outcomes and fixation but caused a high rate of cortical atrophy, proximal stress shielding and bone loss (68–70). The clinical manifestations of this mismatch are thigh pain (60, 61), late presenting avulsion fractures of the greater trochanter (71), an increased susceptibility for periprosthetic femoral fractures and aseptic loosening (27).
The original stems were modified with the application of a more extensive and circumferential porous coating to limit micromotion, by removing the medial surface to reduce the mediolateral bending stiffness, and with the addition of a distal polished bullet tip to prevent pain at the distal part of the stem (72). A prospective study of a cohort of 100 patients performed using this second generation modified stem showed encouraging long term results with 100% survival at a mean follow-up of 11.4 years, and thigh pain in only 2% (72).
McAuley et al (68) in a study of 293 patients under 50 years old using extensively porous-coated CoCr stems matched with beaded, press-fit acetabular components of CoCr or titanium reported survivorship of 96.1% at 15 years follow up. Moyer et al (73) found component survival of 99.1% at a mean of 8.6 years (range 5-10 years) in 115 hips in patients who had a mean age of 39.6 years (range 17-50 years).
The survivorship of these stems has been excellent in long term follow up studies. Thigh pain has been a concern, but the prevalence has been reduced by modifications that decrease the stiffness of second-generation designs. These stems are options for most patients, but studies have not adequately addressed the use of these stems in femora with Dorr Type-C bone.
Proximally coated tapered stems (proximal fixation)
The coating in these implant is typically on the proximal 1/3 segment of the implant and are designed to engage the metaphyseal cortical bone in 1 (medial to lateral), or 2 planes (antero-posterior), and can be considered metaphyseal-filling designs.
Initial stability is obtained by wedge fixation in the metaphysis or by 3-point fixation along the stem length - a straight tapered stem inserted into a femur with an anterior bow. 3-point fixation involves contact between bone and the implant posteriorly, proximally and distally as well as anteriorly in its mid portion.
Preparation requires broaching and no distal reaming. They can be provided with or without a collar to prevent excessive subsidence during ingrowth. A collarless implant allows full seating into the prepared canal (74). Modifications of this design include splines combined with longitudinal slots or flutes to decrease stem stiffness and thigh pain (Fig. 2).
(
Modern metaphyseal type stems have good results in studies and arthroplasty registers with survivorship rates as high as 99% at 15 years (75–78) reported prevalence of thigh pain in up to 12% of patients, but are mild and activity-related (25, 79).
Certain stems utilise a combination of proximal and distal fixation, achieving fixation along the entire length, making them suitable for Dorr Type-C bone. The Synergy porous stem (Smith & Nephew), is a tapered conical straight stem design. It is a third-generation, proximally HA coated implant, with grit blasting in the distal 2/3 and a polished bullet tip. Preparation requires broaches proximally and reaming distally (Fig. 3).
The Synergy porous stem (Smith & Nephew), with a Tapered straight stem design. It is a 3rd-generation, proximally HA coated, tapered uncemented stem. It is made from a titanium alloy and comes in both a standard and high offset version. It is modular with a circulotrapezoidal neck design resulting in a high head-neck ratio. Proximal flutes provide additional rotational stability. (
The results with the use of this stem have been good, with 99.5% survivorship at 75 months in 193 patients (80). Thigh pain was reported in 2.8%, and intraoperative undisplaced fractures of the proximal femur occurred in 3.3% of patients, with no effect on outcome. Osteolysis was however observed proximally in 10% of patients, but no osteolysis was seen distal to the porous coating (80).
Other successful tapered stems utilise HA coating throughout the length of the prosthesis with longitudinal grooves to provide rotational stability (Fig. 4). The results with these stems have been excellent with a number of studies demonstrating excellent immediate to long-term results (81–84).
(
Røkkum et al (82) found no stem subsidence or loosening in the medium term at 5 years in 94 consecutive hip replacements with fully coated HA tapered stems. In another study of 291 stems inserted, there was only 1 case of failure requiring revision at a mean of 10 years follow-up (83). Froimson et al (81) reported no cases of aseptic loosening of the femoral component in 147 hip replacements at a mean of 11.5 years follow-up with the use of this stem. Subsidence and stress shielding occurred in 5% and 2% of cases, respectively, and was not clinically significant, without any occurrence of distal osteolysis.
A widely used stem in Europe, which obtains 3-point fixation in both the diaphysis and metaphysis, is the rectangular, tapered, conical stem, the Alloclassic Zweymuller stem (Zimmer) (Fig. 5). This is a titanium stem with a roughened grit blasted nonporous surface. This stem does not require reaming of the femur for femoral preparation, but the use of rectangular broaches is essential. There have not been any substantial modifications to the original design.
Alloclassic Zweymuller stem (Zimmer). Conical straight stem with rectangular cross section for distal fixation. The titanium included surface roughness with grit-blasted 3 μm-5 μm nonporous surface, included easily accessible extraction holes proximally.
The Zweymuller stem has excellent long-term survivorship, achieving 98% survival of the stem at 15-17 years in some studies (77, 85). Proximal stress shielding was observed in up to 33% of patients in one study (77) and indicates a relatively more distal femoral loading pattern.
Anatomic stems (proximal fixation)
Anatomical stem designs incorporated features to achieve contact over the entire ingrowth surface, and minimise proximal stress shielding (86). The stems are curved and bow posteriorly in the metaphyseal segment but anteriorly in the diaphyseal segment, matching the proximal femoral endosteal geometry (87, 88).
The femoral component was therefore designed to transfer the load over the largest available area with emphasis on priority areas in the metaphysis, it also increases initial press-fit stability with anteversion of the neck built in for separate right and left components (27, 87, 89)
Anatomical variation in the curvature of the femur requires distal reaming and metaphyseal broaching to allow a more precise fit between the prosthesis and the channel prepared. Distally the stems are either tapered or cylindrical and if the tip of the stem is eccentrically placed it will impinge on the anterior cortex.
Kim et al (90) reviewed the results of anatomical stems in 471 patients (601 hips) with a mean follow up of 8.8 years. No patients complained of thigh pain and no patients required revision of their prosthesis (90).
Preparation and implantation technique is therefore less forgiving with these stems, and have been associated with a higher rate of failure and thigh pain in many series (91–95). Kim et al (96) found a clinical failure rate of 9% (eleven of 116 hips) and thigh pain in 28% (32 of 116 hips) at a mean of 6 years.
Customised femoral implants have been introduced in difficult cases with congenital dislocation (99, 100), hip deformities and osteoarthritis secondary to hip dysplasia (97, 98), producing bespoke solutions for these patients. Anatomical variations within the hip joint include extramedullary factors including the anteversion, femoral neck offset and the neck-shaft angle, as well and the intramedullary canal shape (101).
Akbar et al (97) in 72 hip replacements of young patients with a mean age of 35, and osteoarthritis secondary to a femoral deformity, reported 100% survival at 14 years follow up. The use of customised implants in patients without femoral deformity has also been encouraging with high levels of patient satisfaction, functional improvement and absence of revision up to 10 years has been noted in patients without femoral deformity (101).
In 1 series there was an overall survival of 97.3% at 13 years follow up in patients with and without anatomical abnormalities (102). Although there are relatively few studies on the use of uncemented custom femoral components, the results in the mid to long-term follow-up have been encouraging (97, 100, 103–105).
Short stems (proximal fixation)
Almost all short stems are designed for cementless fixation and are intended for younger patients with good bone stock, and in whom the use of a true bone conserving implant without the worries of the potential devastating complications reported after metal-on-metal hip resurfacing arthroplasty (HRA) (106–109). Primary stability, as in conventional THR, is the key element for a good clinical and radiological result (11).
Primary stability in short stems requires control of both rotational and axial forces. Rotational stability is achieved by the bone-metal contact at the level of the diaphysis (11). Short stem implants rely heavily on a sufficient neck cut to provide rotational and axial stability by preserving the femoral neck (110). Nowadays, a common feature of a short stem technique is the use of bone resection to a level just below the femoral head in order to preserve as much of the femoral neck as possible.
The limited surface area for bone-metal contact requires a very effective design to achieve the same stability as for longer implants. Collectively, these stems have not had long-term follow-up. Many newer short designs are modifications of 1 of the above stems, and have no more than 2 years of follow-up (75, 111, 112). In some studies however, survivorship has been comparable to those provided by a cemented THR (113–115). Longer-term follow up studies are needed to demonstrate longevity and efficacy of these stems.
Modular stem (proximal and distal fixation)
In certain circumstances it may be impossible to fit the femur and allow for an adequate stable fixation with a standard implant. In conditions with femoral shaft deformity and malalignment, bone loss, congenital conditions, fractures, or revision surgery it may be necessary to independently prepare the metaphysis and diaphysis.
Modular stems are designed to adequately restore the position of the femoral head and achieve precise fit of the metaphyseal sleeve with separate fixation of the stem in the diaphysis (Fig. 1).
The S-ROM implant (DePuy Orthopaedics) is the most popular cementless, modular, cylindrical implant. Preparation of the stem requires diaphyseal reaming for the stem to obtain cortical contact. Proximally, the modular sleeve fills the proximal femoral metaphysis and accepts the cylindrical stem through its aperture (116). The stem comes with standard and calcar replacement height choices and offset options. This allows the surgeon to decide the anteversion of the femoral component independent of the position of best fit and fill of the proximal femur.
The S-ROM stems are good options for cases with abnormal anatomy. In a study of 55 patients with anatomical abnormalities of the hip, at a mean 10-year follow-up it demonstrated excellent clinical and radiological results with no evidence of radiological loosening and no migration of the femoral implant (117).
Modular implants are not commonly utilised in primary THR. Good short-term and intermediate-term results have been achieved using the S-ROM femoral prosthesis (118). In a large series, Cameron et al reported revision rates of less than 0.5%, femoral loosening rate of 0.25%, and thigh pain in 1.8% in a study of 795 primary THR with a mean of 11 years follow-up (range 2-7 years) (119, 120).
Modular implants are more expensive as they constructed from multiple combinations of proximal and distal segments, requiring a larger inventory of components. While most studies of the use of these stems in primary arthroplasty did not address bone type, the implants have been used in Dorr Type-C femora (11).
Conclusions
Current designs of uncemented femoral stem have seen a number of modifications since their initial introduction and perform well, with good long-term survivorship and functional outcome. Younger patients have greater demands on the function of their hip, and cementless femoral fixation has demonstrated durable results in this population, as well as promising results in older patients.
Limitations in the current literature make it difficult to assess and compare different designs to determine optimal indications for each type. It seems that more than 1 design philosophy allows good performance of the uncemented stem and correct patient selection, and surgical technique are paramount to achieving good results.
Biological fixation, in which the prosthesis is directly fixed to the bone, is the preferred fixation method. Innovations in the future that will address material properties implant geometry and design will undoubtedly improve survivorship and reduce complications associated with cementless femoral stem designs. It is a matter of research and development that will lead to better, more effective and less intrusive THR prostheses.
Future studies of cementless implants should consistently address patient age, activity level, bone type, and deformities so that more definitive conclusions can be drawn about when to use each design.
Footnotes
Financial support: None.
One of the author (Fares S. Haddad) receives Royalties from Smith & Nephew and he receives Institutional and Research Support from: Smith & Nephew, Stryker, Corin, MatOrtho.