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Femoral stem modularity
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Review article
Femoral stem modularity Patrice Mertl ∗ , Massinissa Dehl Service d’orthopédie-traumatologie, CHU d’Amiens, Site Sud, 80054 Amiens Cedex, France
a r t i c l e
i n f o
Article history: Received 26 February 2019 Accepted 13 May 2019 Available online xxx Keywords: Total hip replacement Femoral revision Modularity Modular neck Modular femoral stem
a b s t r a c t Femoral stem modularity in hip replacement was first developed to connect a ceramic head to the stem, then extended to metal heads using the Morse taper principle. Is it a good thing, or a necessary evil? It contributes to improving lower limb length and lateralization setting, at the cost of fairly rare complications such as dissociation and fretting corrosion, which can exceptionally lead to ARMD (Adverse Reaction to Metal Debris). Modular necks were later recommended, with a double Morse taper: cylindrical for the head junction, and more or less flattened for the stem. Is this one modularity too far? Dual modularity in theory perfectly reproduces the biomechanical parameters of the hip, but is unfortunately associated with fractures and severe corrosion, leading to ARMD and pseudotumor, especially in Cr-Co necks. Moreover, it provides no functional advantage, and no longer has a role outside dysplasia and other femoral deformities. Metaphyseal-diaphyseal modularity is not widespread in primary implants, and is it really necessary? Only one model has been widely studied: S–RomTM (Depuy® ). It features a metaphyseal sleeve adapting to the anatomy of the proximal femur, with a stem fitted via an inverse Morse taper. Its only interest is in case of congenital dislocation; like all metal connections, it incurs a risk of fracture and corrosion. On the other hand, modularity is widely employed in revision implants. Does it really help these procedures? The connection between a proximal femoral component of variable geometry and a diaphyseal stem with press-fit distal fixation provides a real solution to problems of length, lateralization and anteversion. Early models encountered high rates of fracture, but current implants and rigorous surgical technique have reduced this risk. Corrosion is a less serious problem, as the Morse taper undergoes only axial stress, without the friction undergone by other models subject to varus stress. © 2019 Elsevier Masson SAS. All rights reserved.
1. Introduction The aim of primary, like revision, total hip replacement (THR) is to restore the biomechanical parameters of an ideal hip so as to optimize functional results. For this, numerous industrial innovations theoretically allow intraoperative adjustment of length, offset and anteversion [1] and, in revision procedures, metaphyseal and diaphyseal filling. The first form of modularity consisted in heads of varying depth to adjust the length of the femoral neck, to which they were attached by a Morse taper. This was then successively extended to: proximal modularity (modular neck) with head-neck and head-metaphysis junctions; metaphyseal modularity; and distal modularity, mainly used in revision implants. Connection between the modular parts is by conical or cylindrical interlocking, with or without screwing. These connections, of which there may be sev-
∗ Corresponding author. E-mail addresses: [email protected], [email protected] (P. Mertl).
eral for a single stem, incur risk of fracture and of corrosion, with the danger, long underestimated, of leading to ARMD (Adverse Reaction to Metal Debris). As early as 1995, Chmell [2] reported elevated rates of radiolucency, osteolysis and aseptic loosening, which he attributed to defective geometry connection. These serious risks motivated the present update on primary and revision modular stems, addressing the following questions: • What are the mechanical principles of femoral head modularity? Is it a good thing, or a necessary evil? • Is neck modularity one modularity too far? • Is metaphyseal-diaphyseal modularity really necessary in a primary implant? • Does modularity help revision? 2. What are the mechanical principles of femoral head modularity? Is it a good thing, or a necessary evil? Stephen A. Morse invented the eponymous taper in 1864 to achieve reliable connection between the rotating parts of a drill
https://doi.org/10.1016/j.otsr.2019.05.019 1877-0568/© 2019 Elsevier Masson SAS. All rights reserved.
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patients; and (3) to monitor progression of corrosion over time. The others were grade D: cleaning the taper, using identical metals, and pairing components of the same brand. Greater head diameter has been implicated in increased headneck interface corrosion. In a finite elements analysis comparing 28, 32, 36 and 44 mm heads, Norman et al. [12] demonstrated that torsion force increased with increasing head diameter. However, head-neck interface corrosion impact seems mainly to concern hard-hard bearings [13] while, in a recent study, Triantaffylopoulos et al. [14] showed that head diameter did not affect the head-neck junction in metal-polyethylene bearings. To sum up: • The usefulness of modular heads is established, at the cost of a very low rate of specific complications; moreover, we are not aware of any monoblock femoral stems on the market; • Increased head diameter calls for caution, especially in metal heads (Cr-Co head on titanium stem), which show greater fretting corrosion and metal debris release. Fig. 1. Principle of the Morse taper.
[3]. The principle consists of a male taper segment (trunnion) onto which a female taper segment (bore) is fixed. Its parameters comprise taper angle, corresponding to slope (e.g. 5◦ .43”), base and summit diameters (usually 12 and 14, or more rarely 10 and 12), and length (Fig. 1 Fig. 1). The female part angle is more obtuse, enabling impaction on the male part; fixation is improved by micro-threading which is crushed under the impaction force. Taper angle and roughness vary between manufacturers, and parts coming from different producers should not be assembled together. The principle was applied in the 1970s to fixing modular heads onto femoral stems–firstly ceramic heads, promoted by Mittlemeier [4], then metal heads. Surgeons could thus adjust neck length intraoperatively, and manufacturers could reduce their range of femoral stems. Modularity incurs a very rare risk of mechanical complications, such as disassembly during dislocation reduction or taper fracture, reported by Barrack as early as 1993 [5]; above all, assembling 2 metal parts, especially when of different compositions, entails corrosion [6], induced by implant micro-motion, with the potentially disastrous long-term consequences of Adverse Reactions to Metal Debris (ARMD). In an experimental study, Fallahnezhad et al. [7] showed that head-neck junction resistance to axial and torsion forces was enhanced by reducing the difference in taper angles and by using Ti/Ti rather than Cr-Co/Cr-Co. It was recently shown that 7.6 joint fluid pH promoted head-neck adhesion [8]; there are thus patientspecific corrosion factors, apart from the implant’s mechanical characteristics. Good quality cleaning and careful drying before assembly also improve fixation and reduces fretting corrosion, for both metal and ceramic heads [9]. Assembly impaction force affects fixation quality, reducing the risk of micro-motion. Under experimental conditions, Panagiotidou et al. [10] compared impaction forces of 2.4 and 8 kN; 8 kN induced greater deformation of implant surface roughness, which is a stabilization factor, and significantly reduced fretting corrosion at 5 millions simulation cycles. In a 2017 systematic literature review of 91 out of 1,224 articles, Wight et al. [11] formulated recommendations to minimize corrosion risk in modular heads. Only 3 of these recommendations were grade B, with good level of evidence: (1) not to use heads causing large offset; (2) to avoid modularity in obese and very active
3. Is neck modularity one modularity too far? Modular necks remain controversial. They have been used for more than 25 years, theoretically providing easy solutions to the 3 major problems of hip surgery: limb-length equalization, whatever the individual anatomy; reproduction of native offset, restoring good soft-tissue tension; and reducing cam effect to achieve greater stability, with unrestricted range of motion and activity [15]. These dual modularity necks have a Morse taper for connection to the head and a flattened taper to connect with the femoral stem (Fig. 2 Fig. 2). They differ in femoral stem connection, geometry and metal alloy. The first model of modular neck had a flat stem connection; manufacturers then developed cylindrical or oval connections, with problems of female component size on the femoral pivot, with risk of fracture [16]. Some cases of stem-cone disjunction were reported [17], but the two main complications are corrosion and fracture [18]. Fracture risk factors have been well identified [19]: obesity, high activity level, large diameter metal-metal bearing, long and varus (CCD angle < 135◦ ) and/or ante- or retroverted modular neck, and corrosion. Cr-Co modular necks had greater fracture resistance (ABG® stem in France, Rejuvenate in the USA), but showed greater corrosion when used with a non-cemented titanium stem, leading to ARMD [20], and were withdrawn from the market. In a systematic review of the literature and of English-language registries, Mihalko et al. [21] showed that stems with modular necks had significantly poorer 10-year survival than stems with simple head modularity, with cumulative revision rates between 4% and 9%. The increased number of femoral component connections increases the quantity of metal debris in peri-articular soft tissue [22], mostly deriving from the taper-stem interface, as mechanical forces differ between proximal (pure compression) and distal connections (varying neck length and body weight inducing a force moment with underlying traction at the lateral part of the taper and compression in the medial part) [23]. Thus, Grupp et al. [24] reported that fatigue fracture always began in the anterolateral region in the upper part of the taper. Intraoperative modular taper assembly conditions depend directly on the surgeon and are thus variable, and likely the main risk factor for complications. Like for the taper receiving the head, it is essential that the modular neck be inserted into a thoroughly cleaned and dry space; Jauch and Grupp [25] demonstrated the great danger of intraoperative contamination of the taper and lodge. Using a hip simulator, they compared micro-motion, which is a source of passive corrosion, in dry titanium and Cr-Co necks
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Fig. 2. Types of modular necks.
versus identical necks contaminated by blood, serum or bone particles; contamination increased micro-motion more than 3-fold in titanium necks and 1.5-fold in Cr-Co necks. Intraoperatively, it is essential to exert the same modular neck impaction force as in the mechanical validation tests. In an experimental study, Aljenai et al. [26] showed that, under a cyclic force of 7 kN, all manually assembled titanium modular necks showed fatigue fracture after a million cycles, whereas those impacted using a hammer did not; conversely, no Cr-Co necks fractured, whatever the assembly technique. Finally, the connection between a Cr-Co modular neck and a Cr-Co stem is associated with higher serum cobalt levels (mean, 1.71 g/l), more elevated with a varus neck [27]. Serum cobalt elevation may be associated with pseudo-tumor, as shown by Kwon et al. [28] in an MRI study with metallic artifact reduction software (MARS) of 148 THRs with Cr-Co modular necks: there were signs of pseudo-tumor in 58 cases, with detection threshold of 3.8 g/l for serum cobalt and 3.8 for Co/Cr ratio. Despite these risks, do modular necks improve functional results? Comparing 436 THRs with modular neck versus standard THR, Carothers et al. [29] found significant differences of more than 4 mm in neck length and more than 2 mm in offset in only 15% of cases. These findings suggest that use of modular heads results in head center positions also achievable with non-modular stems in most cases. There are no studies confirming an impact of modularity on dislocation. In a retrospective series of 809 THRs using the ProfemurTM stem (Wright® ) with modular neck, Gofton et al. [30] reported 2.3% dislocation on a posterior approach, compared to 0.3% on anterior and 0% on lateral approaches, whereas modularity was most often used in posterior approaches. Comparing 2 consecutive series of a single surgeon, with 284 standard and 594 modular neck THRs, Duwelius et al. [31] found no difference in Harris or SF 12 scores at a mean 2.4 years follow-up. On the other hand, modular necks can be useful and simplify surgery in case of dysplasia [32]. A 2018 risk/benefit analysis led the French health products safety agency, ANSM, to draw up strict guidelines for use of modular necks: to be restricted to dysplastic hips or abnormal femurs, to be avoided in case of body-weight > 100 kg, and requiring painstaking cleansing of the modular neck lodge and firm impaction using a 300 g hammer.
To sum up: • Modular necks must imperatively be reserved for difficult cases of dysplasia or severely deformed femur, due to the high risk of fracture; • They are to be avoided in case of body-weight > 100 kg; • Cr-Co necks should be abandoned, in favor of a Ti-Ti connection: Cr-Co/Ti connections are associated with serious corrosion, leading to severe ARMD. 4. Is metaphyseal-diaphyseal modularity really necessary in a primary implant? Few such modular stems have been used in primary procedures. To the best of our knowledge, only the ESOPTM stem (FH® ) had a titanium modular model with an HA-coated metaphyseal component and smooth distal stem connected by a Morse taper (Fig. 3 Fig. 3). In 2008, Garcia-Rey [33] reported 97.8% 7-year survival in a consecutive series of 94 THRs. However, the model was later abandoned. In contrast, the S-RomTM stem (DePuy® ), developed almost 30 years ago, has been widely used in primary hip replacement. It is an uncoated titanium stem with distal fluting, with various offsets, impacted via an inverted Morse taper into a metaphyseal sleeve coated in porous titanium, available in various sizes (Fig. 4 Fig. 4). It has the theoretic advantage of allowing anatomic adaptation of the proximal sleeve into the femoral metaphysis, especially in dysplastic femurs, while also allowing independent adjustment of stem anteversion. Drexler et al. [34] used a 9 mm stem in 30 hypoplastic femurs, with 93.3% 19-year survival. Tudor [35] compared 2 types of proximal sleeve: 17 coated with porous titanium and 102 with HA; 12-year survival was respectively 98.6% and 98.4% (non-significant difference). Wang [36] reported 74 Crowe IV dislocations managed by S-RomTM associated to subtrochanteric osteotomy, with 97.3% 9-year survival. Despite these excellent results, complications specific to this metal connection in primary procedures should not be overlooked. Fabi et al. [37] reported a case of stem/sleeve dissociation causing retroversion leading to dislocation. In a study of 52 S-RomTM explants, Munir [38] found systematic corrosion at the stem/sleeve junction, especially medially, independently of anteversion but
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Fig. 3. ESOPTM modular stem (FH® ).
Fig. 4. S-ROMTM implant (De Puy® ).
aggravated by metal-metal or alumina-alumina bearings, liable to lead to fatigue fracture. However, Yoshitani et al. [39], reporting a case of fracture in S-Rom A, a reduced-size model for the Asian market, found only 4 other cases in the literature. To sum up:
• Metaphyseal-diaphyseal modularity in primary THR is useful only in congenital dislocation; • Corrosion and fracture risks preclude use in simple primary THR.
5. Does modularity help revision? 5.1. Why apply modularity in revision femoral stems? Modular revision implants were developed to avoid the drawbacks of monoblock stems such as the WagnerTM model (Sulzer® ) with tapered cannulated distal fixation, which provided often spectacular bone reconstruction and more than 90% survival, but with a risk of limb-length discrepancy exceeding 1 cm in 14–20% of cases and sometimes exceeding 3 cm, and high dislocation rates between
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7.4% and 14% [40,41]. The rectilinear tapered form provided stable distal blocking, but with no control of its proximal-to-distal location and “automatic” rotation dependent on femoral torsion. Many modular stems were therefore developed, usually with a tapered distal part on the Wagner principle, with variable length and diameter, sometimes curved, and a proximal part of varying length with rotational position set intraoperatively. The proximal part also offers a choice of offsets and types of metaphyseal filling in some models. The definitive implant is assembled either on the surgical table or in-situ, after impacting the distal part into the femur.
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Connection uses a Morse taper, with or without screwing, which is either rectilinear or angled, depending on the model. Distal fixation is by taper press-fit (Fig. 5 Fig. 5) with contact between femur and implant along at least 20 to 50 mm, depending on the report [42]. At least some of the femoral isthmus has to be spared, and thus the technique is in theory inapplicable in extensive Paprosky IIIB and IV bone destruction [43]. For fixation over only 2 cm, an experimental study by Pierson et al. [44] showed that axial and rotational stability was improved by increased taper slope in the distal part of the stem and increased fluting ridges. However, due to the variable sagittal curvature of the femur, distal press-fit contact often comprises only 3 points. Once distal fixation has been achieved, modular proximal parts can be selected so as to precisely restore anteversion, offset and length. Respreto [45], in a study of 122 revision THRs using the RestorationTM modular stem (Stryker® ) reported correction of offset in 66% of cases and of length discrepancy in 78%, with distal bone fixation achieved systematically. Dou [46], in a retrospective comparison of 39 MPTM modular stems and 40 Wagner SLTM stems (Link® ), showed that the former reduced rates of postoperative length discrepancy, with 41% correction, versus 27% with the non-modular model.
5.2. Does modularity incur specific complications in THR revision? Modularity thus allows adaptation to all intraoperatively encountered situations, but with the risks of fracture and corrosion inherent to metallic connections; the earliest models showed high rates of metaphyseal-diaphyseal Morse taper fracture: e.g. PFMRTM (Sulzer® ), with 29% fracture at 14 years according to Roche [47]; 18% fracture or disassembly for Type I ExtrêmeTM (Amplitude® ),
Fig. 5. Distal fixation of tapered stem in femoral isthmus. From Dumoulin [47].
Fig. 6. Use of long proximal parts to distalize connection. From Finck [54].
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Fig. 7. Modular revision stems: A: MPTM (Link® ), B: ZMRTM (Zimmer® ), C: RestorationTM (Stryker® ).
Table 1 Results in main literature series. Survival with revision for femoral stem aseptic loosening as event. Authors Abdel [55] Weiss [56] Klauser [57] Jang [52] Rieger [51] Jibodh [58] Ovesen [59] Lakstein [42] Shivananthan [60] ZMR Smith [61] Riesgo [62] Dzajaa [63] Stimac [64]
Implant MP and Restoration MP Link MP Link Revitan Revitan ZMR ZMR ZMR 64 Restoration Restoration Restoration Restoration
N cases 519 66 63 47 70 54 125 69 11 yrs 115 161 53 86
FU 10 yrs 6 yrs 10 yrs 8 yrs 4.3 yrs 7 yrs 4 yrs 5 yrs 7.3% 7.9 yrs 6.1 yrs 3 yrs 4.3 yrs
Dislocation 3.8% 19% 3.2% 4.2% 8.6% 9% 6% 3% 6% 0.8% 4.3% 5% 2.3%
Subsidence > 5 mm 2.4% 15% 1.6% 2.12% 14.7% ? 8% 11% 0% 6% 1% 2% 0%
Fracture 0.1% 0% 0% 0% 1.4% 0% 1% 1.4% 94% 0% 0.6% 0% 0%
Survival 96% 98% 95% 86%a 96% 100% 94% 94% 99% 97% 96% 93%
Revision for Paprosky types III and IV. a all-cause revision
with its dual epiphyseal-metaphyseal and metaphyseal-diaphyseal modularity, according to Benoit et al. [48]. To meet this risk, some manufacturers connect the proximal and distal components via a Cr-Co Morse taper on the distal part (e.g., RevitanTM , Zimmer® ). However, the Cr-Co-titanium connection aggravates corrosion, with risk of fatigue fracture at the junction [49,50]. Even so, the implant fracture rate was only 1.4% for Rieger [51] and zero for Zang [52]. The second improvement concerned connection design. The fractures of the ZMRTM implant (Zimmer® ) reported by Lakstein [53] led the manufacturer to increase taper diameter from 14.5 mm to 19 mm for large-diameter stems, although other manufacturers continue to use thinner tapers throughout their range (e.g., RestorationTM (Stryker® ), with a 13.16 mm Morse taper connection). A specific milling technique has been developed for Morse tapers, with grit-blasting to harden the connection. Finally, adding a surface coating on the proximal part in the most recent models reduces stress-shielding and avoids mechanical stress peaks, which lead to fracture, at the proximal/distal junction. Finck [54], in a systematic literature review totaling 24 cases of modular revision
implant fracture, found a systematic association of osseointegration of the distal part and absence of bone support in the non-integrated proximal part; he suggested using proximal parts that are as long as possible, so as to shift the connection into healthy bone (Fig. 6 Fig. 6). Fracture risk is also manufacturer-dependent, and seems to have greatly diminished in recent models (Fig. 7 Fig. 7). Moreover, in the MPTM implant (Link® ) (Table 1 Table 1), which is by no means recent, there have been no reports of fracture at all [55–57]. Four series totaling 312 ZMRTM stems (Zimmer® ) showed an overall fracture rate of 0.6% [42,58–60], while 5 series totaling 559 RestorationTM stems (Stryker® ) showed a rate of 0.3% [55,61–64]. Distal taper press-fit fixation further incurs a risk of subsidence, which is not specific to modular stems. Risk factors were analyzed by Tangsataporn [65] in 99 ZMRTM stems, 13% of which showed ≥ 10 mm subsidence at 1 year. Weight > 80 kg and < 2 cm distal diaphyseal press-fit contact were both highly significant independent factors. Long (235 mm) diaphyseal stems and structural allograft were relative risk factors. Bone loss severity, preor intra-operative femur fracture and stem diameter, on the other
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hand, were not significantly associated with subsidence. Subsidence is also strongly related to surgical technique. Patel et al. [66] reported a 25% rate of subsidence (mean, 10 mm; range, 2–29 mm) in a series of 43 RestorationTM stems implanted between 1999 and 2006, which they attributed to under-dimensioning of the distal stem. All these cases of subsidence concerned revision procedures between 2000 and 2004, and none between 2004 and 2006. The authors stressed the difficulties of the learning curve, and recommended controlling final reaming under fluoroscopy and over-dimensioning the definitive stem diameter by 1 size. 5.3. Have modular revision stems reduced the rate of dislocation in revision and provided satisfactory long-term implant survival? Dislocation rates in the literature vary widely between series for a given stem model, and seem highly dependent on surgical technique. Mean incidence overall in the series collated in Table 1 was 5.9% for a total 1,442 modular stems. In the absence of dual mobility cup, used in none of these series, this mean value is relatively low for revision, but masks large variations: with MPTM (Link® ), Klauser [57] reported a rate of 3.2%, compared to 19% for Weiss [56]; with ZMRTM (Zimmer® ), Lakstein reported 3% [42] and Jibodh 9% [58]; and with RestorationTM (Stryker® ), the rate was only 0.8% for Smith et al. [61] but 5% for Dzaja et al. [63]. At follow-ups ranging between 4 and 11 years (Table 1), survival with revision for aseptic loosening as end-point was very satisfactory, ranging from 93% for Stimac et al. [64] to 100% for Jibodh et al. [58]. Dzaja et al. [63] reported just 3 years’ follow-up, but with 96% survival in revision for Paprosky type III and IV loosening. Jang et al. [52] reported just 86% 8-year survival, but with all-cause revision as event, and no differences between the 3 main types of modular stem used (MPTM , ZMRTM and RestorationTM ). Survival thus seems as good as if not better than with non-modular cemented or noncemented stems. Weiss et al. [67], in a comparative study at a mean 4 years’ follow-up based on the Swedish registry, with 812 MPTM and 1,073 long cemented stems, reported 99% and 95% survival, respectively, with revision for aseptic loosening as event. Huang et al. [68] reported 2 consecutive series of 160 MPTM (Link® ) and 129 Wagner SLTM (Zimmer® ) stems implanted from 2002 to 2014 and 2004 to 2014 respectively; at a mean 8 years’ follow-up, survival with aseptic loosening as event was 97.3% for modular and 99% for non-modular stems (non-significant difference). To sum up: • The modular revision stems currently on the market are now a reasonable alternative in THR revision, on condition that the femoral isthmus be spared; • Surgical technique must be rigorous, especially in the choice of the right combination of metaphyseal and diaphyseal part lengths; • Results, however, are to be compared with those of single cephalic modularity revision stems and above all of locked stems. Disclosure of interest The authors declare that they have no competing interest. Funding sources None. Authors’ contribution P. Mertl: writing, revising of the manuscript. M. Dehl: revising, data collection.
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[50] Wang Q, Parry M, Masri BA, Duncan C, Wang R. Failure mechanisms in CrCoMo modular femoral stem for revision total hip arthroplasty. J Biomed Mater Res B App Biomater 2017;105B:1525–35. [51] Rieger B, Ilchmann T, Bolliger L, Stoffel KK, Zwicky L, Causs M. Mid-term result of revision total hip arthroplasty with an uncemented modular femoral component. Hip 2018;28:84–9. [52] Jang HG, Lee KJ, Min BW, Ye HU, Lim KH. Mid-term results of revision total hip arthroplasty using a modular cementless femoral stem. Hip and Pelvis 2015;27:135–40. [53] Lakstein D, Eliaz N, Levi O, Backstein D, Kosashvili Y, Safir O, et al. Fracture of cementless femoral stems at the mid-stem junction in modular revision hip arthroplasty systems. J Bone Joint Surg Am 2011;93:57–65. [54] Finck B. What can the surgeon do to reduce the risk of junction breakage in modular revision stems? Arthroplasty today 2018;4:306–9. [55] Abdel M, Cottino U, Larson DR, Hanssen AD, Lewallen DG, Berry DJ. Modular fluted tapered stems in aseptic revision total hip arthroplasty. J Bone Joint Surg 2017;99:873–81. [56] Weiss RJ, Beckman MO, Enockson A, Schmalholz A, Stark A. Minimum 5 years follow-up of a cementless, modular, tapered stem in hip revision arthroplasty. J Arthroplasty 2011;26:16–22. [57] Klauser W, Bangert Y, Lubinus P, Kendoff D. J Medium-term follow-up of a modular tapered noncemented titanium stem in revision total hip arthroplasty: a single-surgeon experience. Arthroplasty 2013; 28: 84-88. [58] Jibodh SR, Schwarzkopf R, Anthony SG, Malchau H, Dempsey KE, Estok DM. Revision hip arthroplasty with a modular cementless stem: mid-term follow up. J Arthroplasty 2013;58:1167–72. [59] Ovesen O, Emmeluth C, Hofbauer C, Overgaard S. Revision total hip arthroplasty using a modular tapered stem with distal fixation. J Arthroplasty 2010;25:348–54. [60] Sivananthan S, Lim CT, Narkbunnam R, Harris AS, Huddleston JI, Goodman SB. Revision hip arthroplasty using a modular, cementless femoral stem: intermediate-term follow-up. J Arthroplasty 2017;32:1245–9. [61] Smith MA, Deakin AH, Allen D, Baines J. Midterm outcomes of revision total hip arthroplasty using a modular revision hip component. J Arthroplasty 2016;31:446–51. [62] Riesgo AM, Hochfelder JP, Adler EM, Slover JD, Specht LM, Iorio R. Survivorship and complications of revision total hip arthroplasty with a mild-modular femoral stem. J Arthroplasty 2015;30:2260–3. [63] Dzaja I, Lyons MC, McCalden MW, Naudie DD, Howard JL. Revision hip arthroplasty using a modular revision hip system in case of severe bone loss. J Arthroplasty 2014;29:1594–7. [64] Stimac JD, Boles J, Parkes N, Della Valle AG, Boettner F, Vestrich G. Revision total hip arthroplasty with modular femoral stems. J Arthroplasty 2014;29:2167–70. [65] Tangsataporn S, Safir OA, Vincent AD, Abdelbary H, Gross AE, Kuzyck PR. Risk factors of subsidence of a modular tapered femoral stem used for revision total hip arthroplasty. J Arthroplasty 2015;30:1030–4. [66] Patel PD, Klika AK, Murray TG, Elsharkawi KA, Krebs VE, Barsoum WK. Influence of technique with distally fixed modular stems in revision total hip arthroplasty. J Arthroplasty 2010;25:926–31. [67] Weiss RJ, Stark A, Karholm J. A modular cementless stem vs cemented long stem prostheses in revision hip surgery. Acta Orthopaedica 2011;82:136–42. [68] Huang Y, Zhou Y, Shao H, Gu J, Tang H, Tang Q. What is the difference between modular and non modular tapered fluted titanium stems in revision total hip arthroplasty. J Arthroplasty 2017;32:3108–13.
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Review article
Femoral stem modularity Patrice Mertl ∗ , Massinissa Dehl Service d’orthopédie-traumatologie, CHU d’Amiens, Site Sud, 80054 Amiens Cedex, France
a r t i c l e
i n f o
Article history: Received 26 February 2019 Accepted 13 May 2019 Available online xxx Keywords: Total hip replacement Femoral revision Modularity Modular neck Modular femoral stem
a b s t r a c t Femoral stem modularity in hip replacement was first developed to connect a ceramic head to the stem, then extended to metal heads using the Morse taper principle. Is it a good thing, or a necessary evil? It contributes to improving lower limb length and lateralization setting, at the cost of fairly rare complications such as dissociation and fretting corrosion, which can exceptionally lead to ARMD (Adverse Reaction to Metal Debris). Modular necks were later recommended, with a double Morse taper: cylindrical for the head junction, and more or less flattened for the stem. Is this one modularity too far? Dual modularity in theory perfectly reproduces the biomechanical parameters of the hip, but is unfortunately associated with fractures and severe corrosion, leading to ARMD and pseudotumor, especially in Cr-Co necks. Moreover, it provides no functional advantage, and no longer has a role outside dysplasia and other femoral deformities. Metaphyseal-diaphyseal modularity is not widespread in primary implants, and is it really necessary? Only one model has been widely studied: S–RomTM (Depuy® ). It features a metaphyseal sleeve adapting to the anatomy of the proximal femur, with a stem fitted via an inverse Morse taper. Its only interest is in case of congenital dislocation; like all metal connections, it incurs a risk of fracture and corrosion. On the other hand, modularity is widely employed in revision implants. Does it really help these procedures? The connection between a proximal femoral component of variable geometry and a diaphyseal stem with press-fit distal fixation provides a real solution to problems of length, lateralization and anteversion. Early models encountered high rates of fracture, but current implants and rigorous surgical technique have reduced this risk. Corrosion is a less serious problem, as the Morse taper undergoes only axial stress, without the friction undergone by other models subject to varus stress. © 2019 Elsevier Masson SAS. All rights reserved.
1. Introduction The aim of primary, like revision, total hip replacement (THR) is to restore the biomechanical parameters of an ideal hip so as to optimize functional results. For this, numerous industrial innovations theoretically allow intraoperative adjustment of length, offset and anteversion [1] and, in revision procedures, metaphyseal and diaphyseal filling. The first form of modularity consisted in heads of varying depth to adjust the length of the femoral neck, to which they were attached by a Morse taper. This was then successively extended to: proximal modularity (modular neck) with head-neck and head-metaphysis junctions; metaphyseal modularity; and distal modularity, mainly used in revision implants. Connection between the modular parts is by conical or cylindrical interlocking, with or without screwing. These connections, of which there may be sev-
∗ Corresponding author. E-mail addresses: [email protected], [email protected] (P. Mertl).
eral for a single stem, incur risk of fracture and of corrosion, with the danger, long underestimated, of leading to ARMD (Adverse Reaction to Metal Debris). As early as 1995, Chmell [2] reported elevated rates of radiolucency, osteolysis and aseptic loosening, which he attributed to defective geometry connection. These serious risks motivated the present update on primary and revision modular stems, addressing the following questions: • What are the mechanical principles of femoral head modularity? Is it a good thing, or a necessary evil? • Is neck modularity one modularity too far? • Is metaphyseal-diaphyseal modularity really necessary in a primary implant? • Does modularity help revision? 2. What are the mechanical principles of femoral head modularity? Is it a good thing, or a necessary evil? Stephen A. Morse invented the eponymous taper in 1864 to achieve reliable connection between the rotating parts of a drill
https://doi.org/10.1016/j.otsr.2019.05.019 1877-0568/© 2019 Elsevier Masson SAS. All rights reserved.
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patients; and (3) to monitor progression of corrosion over time. The others were grade D: cleaning the taper, using identical metals, and pairing components of the same brand. Greater head diameter has been implicated in increased headneck interface corrosion. In a finite elements analysis comparing 28, 32, 36 and 44 mm heads, Norman et al. [12] demonstrated that torsion force increased with increasing head diameter. However, head-neck interface corrosion impact seems mainly to concern hard-hard bearings [13] while, in a recent study, Triantaffylopoulos et al. [14] showed that head diameter did not affect the head-neck junction in metal-polyethylene bearings. To sum up: • The usefulness of modular heads is established, at the cost of a very low rate of specific complications; moreover, we are not aware of any monoblock femoral stems on the market; • Increased head diameter calls for caution, especially in metal heads (Cr-Co head on titanium stem), which show greater fretting corrosion and metal debris release. Fig. 1. Principle of the Morse taper.
[3]. The principle consists of a male taper segment (trunnion) onto which a female taper segment (bore) is fixed. Its parameters comprise taper angle, corresponding to slope (e.g. 5◦ .43”), base and summit diameters (usually 12 and 14, or more rarely 10 and 12), and length (Fig. 1 Fig. 1). The female part angle is more obtuse, enabling impaction on the male part; fixation is improved by micro-threading which is crushed under the impaction force. Taper angle and roughness vary between manufacturers, and parts coming from different producers should not be assembled together. The principle was applied in the 1970s to fixing modular heads onto femoral stems–firstly ceramic heads, promoted by Mittlemeier [4], then metal heads. Surgeons could thus adjust neck length intraoperatively, and manufacturers could reduce their range of femoral stems. Modularity incurs a very rare risk of mechanical complications, such as disassembly during dislocation reduction or taper fracture, reported by Barrack as early as 1993 [5]; above all, assembling 2 metal parts, especially when of different compositions, entails corrosion [6], induced by implant micro-motion, with the potentially disastrous long-term consequences of Adverse Reactions to Metal Debris (ARMD). In an experimental study, Fallahnezhad et al. [7] showed that head-neck junction resistance to axial and torsion forces was enhanced by reducing the difference in taper angles and by using Ti/Ti rather than Cr-Co/Cr-Co. It was recently shown that 7.6 joint fluid pH promoted head-neck adhesion [8]; there are thus patientspecific corrosion factors, apart from the implant’s mechanical characteristics. Good quality cleaning and careful drying before assembly also improve fixation and reduces fretting corrosion, for both metal and ceramic heads [9]. Assembly impaction force affects fixation quality, reducing the risk of micro-motion. Under experimental conditions, Panagiotidou et al. [10] compared impaction forces of 2.4 and 8 kN; 8 kN induced greater deformation of implant surface roughness, which is a stabilization factor, and significantly reduced fretting corrosion at 5 millions simulation cycles. In a 2017 systematic literature review of 91 out of 1,224 articles, Wight et al. [11] formulated recommendations to minimize corrosion risk in modular heads. Only 3 of these recommendations were grade B, with good level of evidence: (1) not to use heads causing large offset; (2) to avoid modularity in obese and very active
3. Is neck modularity one modularity too far? Modular necks remain controversial. They have been used for more than 25 years, theoretically providing easy solutions to the 3 major problems of hip surgery: limb-length equalization, whatever the individual anatomy; reproduction of native offset, restoring good soft-tissue tension; and reducing cam effect to achieve greater stability, with unrestricted range of motion and activity [15]. These dual modularity necks have a Morse taper for connection to the head and a flattened taper to connect with the femoral stem (Fig. 2 Fig. 2). They differ in femoral stem connection, geometry and metal alloy. The first model of modular neck had a flat stem connection; manufacturers then developed cylindrical or oval connections, with problems of female component size on the femoral pivot, with risk of fracture [16]. Some cases of stem-cone disjunction were reported [17], but the two main complications are corrosion and fracture [18]. Fracture risk factors have been well identified [19]: obesity, high activity level, large diameter metal-metal bearing, long and varus (CCD angle < 135◦ ) and/or ante- or retroverted modular neck, and corrosion. Cr-Co modular necks had greater fracture resistance (ABG® stem in France, Rejuvenate in the USA), but showed greater corrosion when used with a non-cemented titanium stem, leading to ARMD [20], and were withdrawn from the market. In a systematic review of the literature and of English-language registries, Mihalko et al. [21] showed that stems with modular necks had significantly poorer 10-year survival than stems with simple head modularity, with cumulative revision rates between 4% and 9%. The increased number of femoral component connections increases the quantity of metal debris in peri-articular soft tissue [22], mostly deriving from the taper-stem interface, as mechanical forces differ between proximal (pure compression) and distal connections (varying neck length and body weight inducing a force moment with underlying traction at the lateral part of the taper and compression in the medial part) [23]. Thus, Grupp et al. [24] reported that fatigue fracture always began in the anterolateral region in the upper part of the taper. Intraoperative modular taper assembly conditions depend directly on the surgeon and are thus variable, and likely the main risk factor for complications. Like for the taper receiving the head, it is essential that the modular neck be inserted into a thoroughly cleaned and dry space; Jauch and Grupp [25] demonstrated the great danger of intraoperative contamination of the taper and lodge. Using a hip simulator, they compared micro-motion, which is a source of passive corrosion, in dry titanium and Cr-Co necks
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Fig. 2. Types of modular necks.
versus identical necks contaminated by blood, serum or bone particles; contamination increased micro-motion more than 3-fold in titanium necks and 1.5-fold in Cr-Co necks. Intraoperatively, it is essential to exert the same modular neck impaction force as in the mechanical validation tests. In an experimental study, Aljenai et al. [26] showed that, under a cyclic force of 7 kN, all manually assembled titanium modular necks showed fatigue fracture after a million cycles, whereas those impacted using a hammer did not; conversely, no Cr-Co necks fractured, whatever the assembly technique. Finally, the connection between a Cr-Co modular neck and a Cr-Co stem is associated with higher serum cobalt levels (mean, 1.71 g/l), more elevated with a varus neck [27]. Serum cobalt elevation may be associated with pseudo-tumor, as shown by Kwon et al. [28] in an MRI study with metallic artifact reduction software (MARS) of 148 THRs with Cr-Co modular necks: there were signs of pseudo-tumor in 58 cases, with detection threshold of 3.8 g/l for serum cobalt and 3.8 for Co/Cr ratio. Despite these risks, do modular necks improve functional results? Comparing 436 THRs with modular neck versus standard THR, Carothers et al. [29] found significant differences of more than 4 mm in neck length and more than 2 mm in offset in only 15% of cases. These findings suggest that use of modular heads results in head center positions also achievable with non-modular stems in most cases. There are no studies confirming an impact of modularity on dislocation. In a retrospective series of 809 THRs using the ProfemurTM stem (Wright® ) with modular neck, Gofton et al. [30] reported 2.3% dislocation on a posterior approach, compared to 0.3% on anterior and 0% on lateral approaches, whereas modularity was most often used in posterior approaches. Comparing 2 consecutive series of a single surgeon, with 284 standard and 594 modular neck THRs, Duwelius et al. [31] found no difference in Harris or SF 12 scores at a mean 2.4 years follow-up. On the other hand, modular necks can be useful and simplify surgery in case of dysplasia [32]. A 2018 risk/benefit analysis led the French health products safety agency, ANSM, to draw up strict guidelines for use of modular necks: to be restricted to dysplastic hips or abnormal femurs, to be avoided in case of body-weight > 100 kg, and requiring painstaking cleansing of the modular neck lodge and firm impaction using a 300 g hammer.
To sum up: • Modular necks must imperatively be reserved for difficult cases of dysplasia or severely deformed femur, due to the high risk of fracture; • They are to be avoided in case of body-weight > 100 kg; • Cr-Co necks should be abandoned, in favor of a Ti-Ti connection: Cr-Co/Ti connections are associated with serious corrosion, leading to severe ARMD. 4. Is metaphyseal-diaphyseal modularity really necessary in a primary implant? Few such modular stems have been used in primary procedures. To the best of our knowledge, only the ESOPTM stem (FH® ) had a titanium modular model with an HA-coated metaphyseal component and smooth distal stem connected by a Morse taper (Fig. 3 Fig. 3). In 2008, Garcia-Rey [33] reported 97.8% 7-year survival in a consecutive series of 94 THRs. However, the model was later abandoned. In contrast, the S-RomTM stem (DePuy® ), developed almost 30 years ago, has been widely used in primary hip replacement. It is an uncoated titanium stem with distal fluting, with various offsets, impacted via an inverted Morse taper into a metaphyseal sleeve coated in porous titanium, available in various sizes (Fig. 4 Fig. 4). It has the theoretic advantage of allowing anatomic adaptation of the proximal sleeve into the femoral metaphysis, especially in dysplastic femurs, while also allowing independent adjustment of stem anteversion. Drexler et al. [34] used a 9 mm stem in 30 hypoplastic femurs, with 93.3% 19-year survival. Tudor [35] compared 2 types of proximal sleeve: 17 coated with porous titanium and 102 with HA; 12-year survival was respectively 98.6% and 98.4% (non-significant difference). Wang [36] reported 74 Crowe IV dislocations managed by S-RomTM associated to subtrochanteric osteotomy, with 97.3% 9-year survival. Despite these excellent results, complications specific to this metal connection in primary procedures should not be overlooked. Fabi et al. [37] reported a case of stem/sleeve dissociation causing retroversion leading to dislocation. In a study of 52 S-RomTM explants, Munir [38] found systematic corrosion at the stem/sleeve junction, especially medially, independently of anteversion but
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Fig. 3. ESOPTM modular stem (FH® ).
Fig. 4. S-ROMTM implant (De Puy® ).
aggravated by metal-metal or alumina-alumina bearings, liable to lead to fatigue fracture. However, Yoshitani et al. [39], reporting a case of fracture in S-Rom A, a reduced-size model for the Asian market, found only 4 other cases in the literature. To sum up:
• Metaphyseal-diaphyseal modularity in primary THR is useful only in congenital dislocation; • Corrosion and fracture risks preclude use in simple primary THR.
5. Does modularity help revision? 5.1. Why apply modularity in revision femoral stems? Modular revision implants were developed to avoid the drawbacks of monoblock stems such as the WagnerTM model (Sulzer® ) with tapered cannulated distal fixation, which provided often spectacular bone reconstruction and more than 90% survival, but with a risk of limb-length discrepancy exceeding 1 cm in 14–20% of cases and sometimes exceeding 3 cm, and high dislocation rates between
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7.4% and 14% [40,41]. The rectilinear tapered form provided stable distal blocking, but with no control of its proximal-to-distal location and “automatic” rotation dependent on femoral torsion. Many modular stems were therefore developed, usually with a tapered distal part on the Wagner principle, with variable length and diameter, sometimes curved, and a proximal part of varying length with rotational position set intraoperatively. The proximal part also offers a choice of offsets and types of metaphyseal filling in some models. The definitive implant is assembled either on the surgical table or in-situ, after impacting the distal part into the femur.
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Connection uses a Morse taper, with or without screwing, which is either rectilinear or angled, depending on the model. Distal fixation is by taper press-fit (Fig. 5 Fig. 5) with contact between femur and implant along at least 20 to 50 mm, depending on the report [42]. At least some of the femoral isthmus has to be spared, and thus the technique is in theory inapplicable in extensive Paprosky IIIB and IV bone destruction [43]. For fixation over only 2 cm, an experimental study by Pierson et al. [44] showed that axial and rotational stability was improved by increased taper slope in the distal part of the stem and increased fluting ridges. However, due to the variable sagittal curvature of the femur, distal press-fit contact often comprises only 3 points. Once distal fixation has been achieved, modular proximal parts can be selected so as to precisely restore anteversion, offset and length. Respreto [45], in a study of 122 revision THRs using the RestorationTM modular stem (Stryker® ) reported correction of offset in 66% of cases and of length discrepancy in 78%, with distal bone fixation achieved systematically. Dou [46], in a retrospective comparison of 39 MPTM modular stems and 40 Wagner SLTM stems (Link® ), showed that the former reduced rates of postoperative length discrepancy, with 41% correction, versus 27% with the non-modular model.
5.2. Does modularity incur specific complications in THR revision? Modularity thus allows adaptation to all intraoperatively encountered situations, but with the risks of fracture and corrosion inherent to metallic connections; the earliest models showed high rates of metaphyseal-diaphyseal Morse taper fracture: e.g. PFMRTM (Sulzer® ), with 29% fracture at 14 years according to Roche [47]; 18% fracture or disassembly for Type I ExtrêmeTM (Amplitude® ),
Fig. 5. Distal fixation of tapered stem in femoral isthmus. From Dumoulin [47].
Fig. 6. Use of long proximal parts to distalize connection. From Finck [54].
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Fig. 7. Modular revision stems: A: MPTM (Link® ), B: ZMRTM (Zimmer® ), C: RestorationTM (Stryker® ).
Table 1 Results in main literature series. Survival with revision for femoral stem aseptic loosening as event. Authors Abdel [55] Weiss [56] Klauser [57] Jang [52] Rieger [51] Jibodh [58] Ovesen [59] Lakstein [42] Shivananthan [60] ZMR Smith [61] Riesgo [62] Dzajaa [63] Stimac [64]
Implant MP and Restoration MP Link MP Link Revitan Revitan ZMR ZMR ZMR 64 Restoration Restoration Restoration Restoration
N cases 519 66 63 47 70 54 125 69 11 yrs 115 161 53 86
FU 10 yrs 6 yrs 10 yrs 8 yrs 4.3 yrs 7 yrs 4 yrs 5 yrs 7.3% 7.9 yrs 6.1 yrs 3 yrs 4.3 yrs
Dislocation 3.8% 19% 3.2% 4.2% 8.6% 9% 6% 3% 6% 0.8% 4.3% 5% 2.3%
Subsidence > 5 mm 2.4% 15% 1.6% 2.12% 14.7% ? 8% 11% 0% 6% 1% 2% 0%
Fracture 0.1% 0% 0% 0% 1.4% 0% 1% 1.4% 94% 0% 0.6% 0% 0%
Survival 96% 98% 95% 86%a 96% 100% 94% 94% 99% 97% 96% 93%
Revision for Paprosky types III and IV. a all-cause revision
with its dual epiphyseal-metaphyseal and metaphyseal-diaphyseal modularity, according to Benoit et al. [48]. To meet this risk, some manufacturers connect the proximal and distal components via a Cr-Co Morse taper on the distal part (e.g., RevitanTM , Zimmer® ). However, the Cr-Co-titanium connection aggravates corrosion, with risk of fatigue fracture at the junction [49,50]. Even so, the implant fracture rate was only 1.4% for Rieger [51] and zero for Zang [52]. The second improvement concerned connection design. The fractures of the ZMRTM implant (Zimmer® ) reported by Lakstein [53] led the manufacturer to increase taper diameter from 14.5 mm to 19 mm for large-diameter stems, although other manufacturers continue to use thinner tapers throughout their range (e.g., RestorationTM (Stryker® ), with a 13.16 mm Morse taper connection). A specific milling technique has been developed for Morse tapers, with grit-blasting to harden the connection. Finally, adding a surface coating on the proximal part in the most recent models reduces stress-shielding and avoids mechanical stress peaks, which lead to fracture, at the proximal/distal junction. Finck [54], in a systematic literature review totaling 24 cases of modular revision
implant fracture, found a systematic association of osseointegration of the distal part and absence of bone support in the non-integrated proximal part; he suggested using proximal parts that are as long as possible, so as to shift the connection into healthy bone (Fig. 6 Fig. 6). Fracture risk is also manufacturer-dependent, and seems to have greatly diminished in recent models (Fig. 7 Fig. 7). Moreover, in the MPTM implant (Link® ) (Table 1 Table 1), which is by no means recent, there have been no reports of fracture at all [55–57]. Four series totaling 312 ZMRTM stems (Zimmer® ) showed an overall fracture rate of 0.6% [42,58–60], while 5 series totaling 559 RestorationTM stems (Stryker® ) showed a rate of 0.3% [55,61–64]. Distal taper press-fit fixation further incurs a risk of subsidence, which is not specific to modular stems. Risk factors were analyzed by Tangsataporn [65] in 99 ZMRTM stems, 13% of which showed ≥ 10 mm subsidence at 1 year. Weight > 80 kg and < 2 cm distal diaphyseal press-fit contact were both highly significant independent factors. Long (235 mm) diaphyseal stems and structural allograft were relative risk factors. Bone loss severity, preor intra-operative femur fracture and stem diameter, on the other
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hand, were not significantly associated with subsidence. Subsidence is also strongly related to surgical technique. Patel et al. [66] reported a 25% rate of subsidence (mean, 10 mm; range, 2–29 mm) in a series of 43 RestorationTM stems implanted between 1999 and 2006, which they attributed to under-dimensioning of the distal stem. All these cases of subsidence concerned revision procedures between 2000 and 2004, and none between 2004 and 2006. The authors stressed the difficulties of the learning curve, and recommended controlling final reaming under fluoroscopy and over-dimensioning the definitive stem diameter by 1 size. 5.3. Have modular revision stems reduced the rate of dislocation in revision and provided satisfactory long-term implant survival? Dislocation rates in the literature vary widely between series for a given stem model, and seem highly dependent on surgical technique. Mean incidence overall in the series collated in Table 1 was 5.9% for a total 1,442 modular stems. In the absence of dual mobility cup, used in none of these series, this mean value is relatively low for revision, but masks large variations: with MPTM (Link® ), Klauser [57] reported a rate of 3.2%, compared to 19% for Weiss [56]; with ZMRTM (Zimmer® ), Lakstein reported 3% [42] and Jibodh 9% [58]; and with RestorationTM (Stryker® ), the rate was only 0.8% for Smith et al. [61] but 5% for Dzaja et al. [63]. At follow-ups ranging between 4 and 11 years (Table 1), survival with revision for aseptic loosening as end-point was very satisfactory, ranging from 93% for Stimac et al. [64] to 100% for Jibodh et al. [58]. Dzaja et al. [63] reported just 3 years’ follow-up, but with 96% survival in revision for Paprosky type III and IV loosening. Jang et al. [52] reported just 86% 8-year survival, but with all-cause revision as event, and no differences between the 3 main types of modular stem used (MPTM , ZMRTM and RestorationTM ). Survival thus seems as good as if not better than with non-modular cemented or noncemented stems. Weiss et al. [67], in a comparative study at a mean 4 years’ follow-up based on the Swedish registry, with 812 MPTM and 1,073 long cemented stems, reported 99% and 95% survival, respectively, with revision for aseptic loosening as event. Huang et al. [68] reported 2 consecutive series of 160 MPTM (Link® ) and 129 Wagner SLTM (Zimmer® ) stems implanted from 2002 to 2014 and 2004 to 2014 respectively; at a mean 8 years’ follow-up, survival with aseptic loosening as event was 97.3% for modular and 99% for non-modular stems (non-significant difference). To sum up: • The modular revision stems currently on the market are now a reasonable alternative in THR revision, on condition that the femoral isthmus be spared; • Surgical technique must be rigorous, especially in the choice of the right combination of metaphyseal and diaphyseal part lengths; • Results, however, are to be compared with those of single cephalic modularity revision stems and above all of locked stems. Disclosure of interest The authors declare that they have no competing interest. Funding sources None. Authors’ contribution P. Mertl: writing, revising of the manuscript. M. Dehl: revising, data collection.
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Please cite this article in press as: Mertl P, Dehl M. Femoral stem modularity. Orthop Traumatol Surg Res (2019), https://doi.org/10.1016/j.otsr.2019.05.019