INSTRUCTIONAL REVIEW: HIP
Modular neck femoral stems
H. Krishnan, S. P. Krishnan, G. Blunn, J. A. Skinner, A. J. Hart From Royal National Orthopaedic Hospital, Stanmore, United Kingdom
Following the recall of modular neck hip stems in July 2012, research into femoral modularity will intensify over the next few years. This review aims to provide surgeons with an up-to-date summary of the clinically relevant evidence. The development of femoral modularity, and a classification system, is described. The theoretical rationale for modularity is summarised and the clinical outcomes are explored. The review also examines the clinically relevant problems reported following the use of femoral stems with a modular neck. Joint replacement registries in the United Kingdom and Australia have provided data on the failure rates of modular devices but cannot identify the mechanism of failure. This information is needed to determine whether modular neck femoral stems will be used in the future, and how we should monitor patients who already have them implanted. Cite this article: Bone Joint J 2013;95-B:1011–21.
H. Krishnan, BSc(Hons), MBBS, MRCS(Eng), Orthopaedic SpR S. P. Krishnan, D.Orth, MS(Orth), FRCS (Orth), Orthopaedic SpR G. Blunn, PhD, BSc, Professor of Orthopaedics J. A. Skinner, MBBS, FRCS(Eng), FRCS(Orth), Orthopaedic Consultant A. J. Hart, MA, MD, FRCS (Orth), Professor of Orthopaedics, Orthopaedic Consultant Royal National Orthopaedic Hospital, Brockley Hill, Stanmore, Middlesex HA7 4LP, UK. Correspondence should be sent to Mr H. Krishnan; e-mail:
[email protected] ©2013 The British Editorial Society of Bone & Joint Surgery doi:10.1302/0301-620X.95B8. 31525 $2.00 Bone Joint J 2013;95-B:1011–21.
VOL. 95-B, No. 8, AUGUST 2013
Improvements in design features and biomaterials, with the aim of reproducing the natural biomechanics of the hip to maximise both function and longevity in total hip replacement (THR), include modular neck systems. These allow a wide range of adjustment of the centre of rotation of the femoral head. However, there have been problems, and in July 2012 the ABG2 and Rejuvenate modular neck stems (Stryker, Kalamazoo, Michigan) were recalled because of high revision rates as a result of metal debris from the modular junction.1 More than 30 000 THRs with these stems have been implanted worldwide,2 and in the United Kingdom more than 6000 various modular neck stems have been registered with the National Joint Registry.3 The change from monobloc to modular head– neck junctions allows for the adjustment of leg length and horizontal femoral offset.4 Modularity is increased through extra junctions between the neck and stem and the metaphysis and diaphysis,5 permitting independent adjustments to be made in the vertical and horizontal offsets, the leg length and the version of the neck, which is especially advantageous in patients with complex anatomy. The disadvantages of additional modular junctions include an increased risk of mechanical failure and the production of metal debris that may cause adverse local tissue reactions (ALTR).6,7 This review describes the evolution of current modular neck designs so that surgeons can understand the advantages and disadvantages of these stems.
Classification and development of modularity There are three broad types of modularity: proximal, mid-stem and distal.8 Proximal modularity includes head–neck junctions, neck– stem junctions, anterior–posterior pads, modular collars, proximal shoulders and stem sleeves (Fig. 1). McBride9 designed the ‘door-knob’ hip prosthesis, which was an attempt to assemble a spherical head onto a shaft.10 Lippman11 used a modular ‘transfixation hip prosthesis’ comprising a stem with a saddle and end plate, prosthetic head, pivot rod and accessory collars. In 1961, Weber12 developed designs of modularity at the head–neck junction (Fig. 2) in order to allow movement in different planes and to reduce friction and wear. The first trunnionbased design, manufactured in 1968, was a metal neck with a polyethylene head and metal socket12,13; however, the polyethylene head caused excessive wear, leading to granuloma formation.14 In 1971, the design was changed to metal heads with a metal-on-metal combination at the neck and a metal head and polyethylene socket, providing the option of varying neck length.15 Weber’s original design had a ‘little channel’ through the head that allowed fluid inflow to lubricate the head-socket as well as the head–neck cylindrical interface of the trunnion. This was replaced in 1972 by a ceramic 32 mm head with the aim of improving wear at the head–socket interface.16 In 1975 the design was 1011
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H. KRISHNAN, S. P. KRISHNAN, G. BLUNN, J. A. SKINNER, A. J. HART
Fig. 1a
Fig. 1b
Fig. 1c
Fig. 1e
Fig. 1d
Fig. 1f
Images showing various forms of proximal femoral modularity: a) head–neck junctions, b) neck–stem junctions, c) proximal shoulder, d) anterior/ posterior pads, e) modular collar and f) stem–sleeve junction.
First THR (Wiles)
1938
Monobloc THR (Charnley)
1948
1962
Threaded modular femoral (McBride)
Modular head/neck (Weber)
1967 Modular proximal sleeve (Sivash)
1971
1991 Modular neck (Cremascoli)
Fig. 3 Timeline chart of the development of modularity (THR, total hip replacement).
Fig. 2 Original drawing of a trunnion bearing total hip replacement on 23 April 1961, showing the first design of modularity at the head–neck junction (reproduced with permission of B. G. Weber).
enhanced by incorporating a 3° Morse taper, adding a distal coronal slot and eight distal flutes to increase torsional stability,17 and this was used clinically in the United States as the S-ROM (Sivash Range of Motion) prosthesis (United States Surgical Corp., Stamford, Connecticut).18 Subsequently, the distal portion of the stem was polished, and modular heads, a calcar spout and a series of internal collars to enhance bone–implant contact were added. In 1985, the first modular-neck stem (with junctions at head–neck and neck–stem) was developed by Cremascoli (Milan, Italy) – the ANCA-Fit (Fig. 3). The neck–stem taper was oval, giving rotational resistance.19 In the 1990s a modular femoral component with a variety of neck–stem combinations was introduced,20 and over the last three decades different types of modular prosthesis with specific design features have been developed (Table I) (Fig. 4).
Coupling mechanisms Head–neck junction. The modular femoral head–neck junction is frequently referred to as a ‘Morse taper’. The
trunnion (male portion) compresses the bore (female portion). The length, surface roughness and angle of the taper vary with the manufacturer. Larger tapers and those that protrude below the distal aspect of the femoral head risk impingement and limitation of movement.21 Various stems use designs based on the Feldmuhle specifications, ranging from 9/10 to 14/16 taper.22 For example, a 9/10 stem has a conical side wall with a maximum diameter of 10 mm that tapers down to a circular end wall of 9 mm. Dimensional mismatch and material combinations determine relative movement and corrosion at the interface.23,24 Larger tapers were found to cause impingement,25 which led to the development of smaller tapers with larger heads, but with the risk of fatigue fractures and fretting corrosion.26,27 Neck–stem junction. In a modular neck-stem there is a ‘double taper’ as the trunnion of the distal part of the neck engages with a bore created within the stem, as well as the proximal engagement of the neck with the head as described. This junction is subject to both axial and bending stresses, and modular stem–shoulder interfaces use the bore engagement mechanism. A set pin or screw can be used at modular stem–sleeve junctions to prevent rotation of the taper, but there is no evidence to suggest a reduction in torsional fretting.
Advantages of modular junctions Biomechanical. For optimal biomechanics, the centre of rotation of the hip should be reproduced as accurately as possible. This could be achieved by using either custom-made THE BONE & JOINT JOURNAL
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Table I. Properties of modular-neck hips Material‡ Ability to Site of adjust Options offset Length modular† CCD angles (°) Stem Neck shape (n) Version (°) (mm) (mm) ity
Company*
Prosthesis
Aesculap/B Braun Adler Ortho
3 9
130, 135, 140 n/a
8 7.5
n/a 15
10 15
Neck Neck
Ti6Al4V Ti6Al4V
CoCr Ti6Al4V
Smith & Nephew
Metha Elliptical Modular Elliptical supercharged SC Redapt Elliptical
5
125, 137, 131
10
16
10
Ti6Al4V
CoCr
Smith & Nephew
SMF
4
125, 137, 131
10
Ti6Al4V
CoCr
Symbios Wright Medical Technology Wright Medical Technology Wright Medical Technology Wright Medical Technology
SPS modular Elliptical ANCA Fit Elliptical
12 17
n/a 127, 135, 143
15 8, 15
Stand- 8 ard/high n/a n/a 13.5 17
Neck, Stem/ Sleeve Neck Neck Neck
Ti6Al4V Ti6Al4V
CoCr Ti6Al4V/CoCr
Profemur X
Elliptical
12
127, 135, 143
4, 8, 15
13.5
13.5
Neck
CoCr
Ti6Al4V/CoCr
Profemur Gladiator Profemur R
Elliptical
12
127, 135
4,8,15
13.5
13.5
Neck
CoCr
Ti6Al4V/CoCr
Elliptical
12
127, 135, 143
15
13.5
13.5
Ti6Al4V
Ti6Al4V/CoCr
Wright Medical Technology Stryker Stryker WG Healthcare UK Ltd Zimmer
Profemur E
Elliptical
12
127, 135, 143
4, 8, 15
13.5
13.5
Neck, Stem/ Sleeve Neck
Ti6Al4V
Ti6Al4V/CoCr
ABG II Rejuvenate H-Max M/L Taper Kinectiv F2L
Elliptical Elliptical Elliptical Elliptical
10 10 12 32
125, 130, 135 125, 130, 136 131, 134 132, 133, 134
5, 19 5, 20 10 4, 10
10 10 5 8
9.5 9.5 10.5 8
Neck Neck Neck Neck
TiMo12Zr6Fe2 TiMo12Zr6Fe2 Ti6Al4V Ti6Al4V
CoCr CoCr Ti6Al4V Ti6Al4V
Circular
6
125, 135, 145
10
n/a
10
Neck
Ti6Al4V
Ti6Al4V
Lima
Elliptical
Neck
* Aesculap/B Braun (Sheffield, United Kingdom); Adler Ortho (Milan, Italy); Smith & Nephew (Memphis, Tennessee); Symbios (Yverdon, Switzerland); Wright Medical Technology (Arlington, Tennessee); Stryker (Kalamazoo, Michigan); WG Healthcare UK Ltd (Letchworth, United Kingdom); Zimmer (Warsaw, Indiana); Lima (Udinese, Italy) † CCD, collum–caput–diaphysis ‡ Ti, titanium; Al, aluminium; V, vanadium; Co, cobalt; Cr, chromium; Mo, molybdenum; Zr, zirconium; Fe, iron
or computer-assisted-design/-manufacture (CAD-CAM) implants, with predetermined levels of resection of the neck and depth of insertion of the socket,28 or an off-theshelf prosthesis that allows for variations in femoral anatomy,29 including femoral neck length, shaft diameter, the angle between the longitudinal axes of the femoral neck and shaft (also known as the collum–caput– diaphysis (CCD) angle30), version of the neck and offsets.28,29 Restoration of femoral offset and soft-tissue balancing can reduce abductor muscle imbalance, pain and rates of wear. 31 The versatility of modular neckstems allows the leg length to be adjusted independently of the vertical and horizontal femoral offsets. The ability to adjust version and offset is useful in the prevention of impingement between the socket and neck, and bony and muscular impingement.32 In a study of monobloc stems, femoral offset and limb length were not restored in 36% of patients (28 of 79).33 Pre-operative planning with three-dimensional CT and the use of the SPS neck (Symbios, Exeter, United Kingdom) restored offset in 94% of hips (209 of 223) and neck–shaft angle in 93% (207 of 223).34 Miki and Sugano35 investigated the range of movement without prosthetic impingement using CAD models in a ANCA-Fit modular-neck system (Wright Medical Technology, VOL. 95-B, No. 8, AUGUST 2013
Arlington, Tennessee). They showed that modular neck stems could provide an adequate range of movement without impingement in patients with up to 60° of femoral anteversion. Near normal reconstruction of femoral anteversion using modular neck–stems showed an improved range of movement in patients with an excessively anteverted or retroverted femur.36 However, the extent of this correction with modular neck-stems is sometimes limited by the softtissue anatomy.37,38 In patients with > 47° of anteversion, adequate correction could not be achieved using a modular neck–stem system (Artificial Joint System; Cremascoli).37 Satisfactory medium- to long-term clinical results of modular stems in patients with complex anatomy of the proximal femur have been reported.39-43 Mixing of materials. Modularity makes it possible to combine materials in order to optimise fixation, resistance to fracture and wear characteristics of bearing surfaces (for example a combination of a titanium alloy (such as Ti6Al4V) stem, cobalt–chromium (CoCr) neck and a ceramic or cobalt–chromium–molybdenum (CoCrMo) head). Several designs use these combinations (Table I). Common components are either CoCr alloys (such as ASTM F7544 with Co(60%)-Cr(28%)-Mo(6%)) or titanium alloys (such as Ti6Al4V). Uncemented femoral components are usually
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Fig. 4a
Fig. 4e
Fig. 4c
Fig. 4b
Fig. 4f
Fig. 4d
Fig. 4g
Fig. 4h
Fig. 4i
Images showing various examples of modular neck femoral stems: a) DTC Margron modular hip prostheses (Portland Orthopaedics Pty Ltd, Matraville, Australia); b) ABG II (Stryker, Kalamazoo, Michigan); c) Rejuvenate (Stryker); d) Modular supercharged SC (Adler Ortho, Milan, Italy); e) M/L Taper Kinectiv (Zimmer, Warsaw, Indiana); f) ANCA-Fit (Wright Medical Technology, Arlington, Tennessee); g) Profemur E (Wright Medical Technology); h) Profemur Z (Wright Medical Technology); and i) Profemur R (Wright Medical Technology).
titanium alloys, although the exact type varies, which may be clinically significant.45 CoCr is the favoured cemented stem as it is less prone to fretting.46 For the neck component, the advantages of CoCr alloys include resistance to high loading and corrosion. Ti alloys are more resistant to corrosion albeit less resistant to high loading compared with CoCr alloys in general.47 Facilitation of revision arthroplasty. Modularity allows the exchange of proximal components while retaining a wellfixed femoral stem, and reduces morbidity at revision while preserving proximal femoral bone stock. This is relevant when just the head is exchanged at the head–neck junction, for example changing the bearing surfaces for wear, adjusting the neck length, offset, head diameter or constraint to treat recurrent dislocation48,49 and revision of the socket requiring a compatible head. It is also relevant to stems with a metaphyseal modular junction (Fig. 1), such as when the stem has subsided to a stable position and the surgeon needs to change the proximal body and head (type c). It is less relevant, however, for modular neck-stems (type b) because the reason for revision often involves the neck–stem junction.50,51 Facilitation of small-incision surgical techniques. Modularity at the proximal stem allows surgery through either small
single or even smaller double incisions, although there are concerns about early complications following mini-incisions.52 Reduction of implant inventory. Modular head–neck junctions allow the use of a wide variety of head sizes and materials, without requiring a large number of different hip systems. Additional neck–stem modularity allows the use of multiple angles of the neck without the need for additional rasps, trials or definitive stems. For example, the M/L Taper Kinectiv stem (Zimmer, Warsaw, Indiana) allows 32 different combinations of stem, neck and head, but requires only three trays of instrumentation, saving on transport, sterilisation and lifting, and a reduced number of implants.
Clinical applications Primary hip arthroplasty. There is no mathematically dis-
cernible relationship between medial offset and the dimensions of the proximal femoral medulla.53 For example, small women with osteoporosis have a small offset but a large medulla. Modular neck prostheses enable independent adjustment of these two parameters, providing accurate reconstruction of the centre of rotation of the hip54 (Table II),55-63 in contrast to the inadequacy of monobloc components in restoring femoral offset.64 THE BONE & JOINT JOURNAL
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Table II. Clinical outcomes of modular neck prostheses for primary, complex anatomy and revision total hip replacement (THR) Harris hip score Author/s
Prosthesis
Mean age Follow-up Heterotopic Peri-prosthetic Number (yrs) (yrs) Pre-op Post-op Revisions Infection Loosening ossification (%) Dislocation fracture Subsidence
Primary THR 48 Braun et al55 Metha Omlor et al56 Profemur E 155 Dwelius et al57 Zimmer M/L 621 Kinectiv 319 Blakey et al58 ANCA Fit
54 66 63
2.4 9 1.2
51
96 94 91
1 1 6
0 0 2
1 0 0
0 30 0
0 0 6
1 1 1
7 0 0
64
7.2
-
-
12
0
2
0
5
1
1
THR for complex anatomy Traina et al40 Sakai et al39
ANCA Fit 61 Profemur Z 83
49.4 52.9
9.7 14.5
45
74.7 99
0 0
0 0
0 6
0 0
0 0
0 0
0 0
Revision THR Artiaco et al60 Pattyn et al61 Koster et al62 Weng et al63
Profemur R Profemur R Profemur R Profemur R
72 67 73 54.2
5 5.2 6.2 1.1
50 40 25
78 75 72
2 5 3 0
2 1 1 0
0 2 0 0
0 0 0 0
0 3 0 0
0 1 0 3
3 1 2 0
35 68 73 12
Complex anatomy. In hip dysplasia the femoral deformity
includes a short femoral neck with the greater trochanter located above the femoral head, an increased angle of anteversion, a narrow femoral canal,65,66 and potentially deformity of the proximal femur as a result of previous osteotomies. Furthermore, femoral offset progressively decreases as the amount of subluxation increases.67 Obtaining a good fit and fill in the straight and small femoral canal is difficult when a non-modular standard stem is used.68,69 A modular neck–stem has been shown to produce a better outcome and range of movement with reduced rates of osteolysis compared with a standard uncemented stem in patients with dysplasia40 (Table II). A rate of survival of 97.5% at 11 years has been reported for components with a modular neck.41 Revision arthroplasty. The available proximal femoral bone stock in single-stage revision surgery can only be judged after removal of the femoral component. Monoblock femoral components are usually inadequate, as independent adjustments of offset, neck version and stem diameter are often necessary to create a stable joint. The use of revision stems with modularity at the neck–stem and mid-stem junctions allow for fine-tuning, with improved outcomes (Table II).60,62,63,70
Complications Mechanical failure. Insufficient bony support at the calcar, obesity, the design of the components, the materials used and stress risers such as laser etching or manufacturer logos have all been thought to contribute to failure at the proximal stem–neck junction.6,7,71,72 Dangles and Altstetter7 reported the first case of fracture of a modular neck in an obese middle-aged man, and proposed that increased offset of the modular neck and crevice corrosion were responsible for failure. Other reports have shown a common pattern of fractures of the modular neck in obese patients with long femoral necks.6,71,73 VOL. 95-B, No. 8, AUGUST 2013
Skendzel et al71 reported that the ‘long varus’ neck component increases the bending moment by 32.7% compared with the standard ‘short varus’ neck. If the neck is retroverted, this further increases the moment arm and tensile stresses to the anterior lateral corner of the neck.74 High mechanical stress occurs secondary to eccentric loading of the neck–stem junction. A retrieval study of 16 DTC Margron modular hip prostheses (Portland Orthopaedics Pty Ltd, Matraville, Australia) reported that degradation at the neck–stem junction was more significant than at the head– neck junction secondary to the high mechanical stress and increased lever arm.75 The choice of material at the junction can have an effect on the durability and survival of the component. The Profemur (Wright Medical Technology) and Metha Short Hip Systems (Aesculap AG, Tuttlingen, Germany) both used long titanium necks.72 Nganbe et al76 performed an in vitro assessment of the Profemur modular neck comparing Ti6Al4V with CoCrMo alloys. They were able to show a 38% higher load-bearing capacity and 1000 times longer fatigue life of the CoCrMo neck compared with the Ti6Al4V modular necks. However, this study was limited as they were not able to assess the corrosion behaviour at these modular junctions. Grupp et al77 found a rate of failure of 1.4% (68 of 4000 cases) in modular Metha Short Hip System necks over a two-year period. They reported that a change in the adapter material from Ti6Al4V to Co28Cr6Mo significantly improved the safety of the junction. The Co28Cr6Mo neck showed increased rigidity, higher resistance against fretting and enhanced fatigue strength. Cold welding and taper damage at the neck–stem junction. One of the potential advantages of having a modu-
lar neck–stem junction is the ability to revise the head and neck while leaving the stem in situ. However, several cases of cold welding and damage of the taper at the neck–stem junction have been reported, resulting in difficulty in disengaging
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Table III. Material used and mechanisms of failure for the components of modular-neck hips Material† Prosthesis*
Site of modularity
Neck
Stem
Reported failure mechanism
Cases reported (n)
Profemur Z87 Metha Short Hip System76
Neck/stem Neck/stem
Ti6Al4V Ti6Al4V
Ti6Al4V Ti6Al4V
5 68
Adaptor GHE/s short stem88
Neck/stem
CoCr
CoCr
DTC Margron51
Neck shoulder/stem
CoCr
Ti6Al4V
ABG II and Rejuvenate1
Neck/stem
CoCr(GADS) TiMo12Zr6Fe2
Alpha II84
Neck/stem
CoCr
Neck fracture Micromovements at the neck adapter–stem junction resulted in fretting and micro-cracks, ultimately leading to fatigue fracture Corrosion at the neck stem taper causing pseudotumour High mechanical stress due to increased lever arm resulting in degradation of neck–stem junction Excessive fretting and/or corrosion leading to adverse local tissue reaction, hypersensitivity reactions and osteolysis Dissociation of modular femoral neck from the stem
CoCr
3 16
60
1
* Profemur Z (Wright Medical Technology, Arlington, Tennessee); Metha Short Hip System (B Braun, Sheffield, United Kingdom); Adaptor GHE/s short stem (Eska Implants AG, Lubeck, Germany); DTC Margron modular hip prostheses (Portland Orthopaedics Pty Ltd, Matraville, Australia); ABG II and Rejuvenate (Stryker, Kalamazoo, Michigan); Alpha II (Osteoimplant Technology Incorporated, Hunt Valley, Maryland) † Ti, titanium; Al, aluminium; V, vanadium; Co, cobalt; Cr, chromium; Mo, molybdenum; Zr, zirconium; Fe, iron; GADS, Gas Atomised Dispersion Strengthened
the neck from the stem.78-80 Cold welding is a solid-state joining process that occurs without fusion/heating at the interface between the neck and stem junction. Ti6Al4V components are more susceptible to cold welding than CoCrMo components.78 Careful engagement of the neck and stem is crucial during insertion, as excessive force can damage the surface of the coupling. The fretting process (mechanical micromovement) is strongly influenced by the distribution of pressure at the interfaces, and also by the surface roughness and finish.79 Continuous abrasion and repassivation (‘passivation’ is the formation of a protective metal-oxide layer) at the modular junction are thought to be the reasons for cold welding.77 Theoretically, this could reduce fretting and corrosion at this junction. However, when the neck cannot be retrieved the entire neck–stem construct should be revised. A well-fixed stem is often encountered with no part of the stem available to engage with an extraction device. This makes the use of an extended trochanteric osteotomy, with its associated morbidity, almost essential when removing the stem. An in vitro study using the ANCA-Fit modular-neck system (Wright Medical Technology) measured the extraction forces required after different levels of press-fitting of the modular neck to the stem were simulated,80 and found that fewer walking cycles after manual insertion of the coupling are sufficient to give the component high stability. Hammer blows were not recommended, as they could compromise the shape and surface at the neck–stem junction. Dissociation of modular components. Dissociation at the head–neck junction is rare as the forces involved are compressive rather than distractive. It has been reported during attempted closed reduction of a dislocated hip, and following trauma, especially with mismatched head–neck tapers.25 The presence of blood or fat in the modular interface predisposes to disassociation.25,81
Dissociation at the neck–stem junction can include subsidence of the stem, resulting in loss of soft-tissue tension and causing telescopic movement of the neck; improper orientation of the neck on the stem, resulting in stress concentration at the interface; impingement of the femoral neck on the acetabular component; osteophytes and defective material properties, resulting in micromovement; and fretting, leading to failure of the interface.76,78,82,83 Sporer et al84 described a patient with dissociation of a modular femoral neck (Alpha II; Osteoimplant Technology Inc., Hunt Valley, Maryland) and demonstrated that the use of a short and small trunnion in an obese patient could result in deformation of the taper at the neck–stem junction. Dissociation at the stem–sleeve (metaphyseal) junction is rare, but has been reported with S-ROM (Joint Medical Products, Stamford, Connecticut) and Apex stems (Omni Life Science, Raynham, Massachusetts).85 Shearing of the set pin caused failure of the Apex stem. Failure of the Morse taper occurred in one case of S-ROM failure, and a later case failed because of fracture of the proximal stem within the sleeve.85 Corrosion at the modular interface. Mechanical failure of modular necks is thought to occur secondary to crevice corrosion, which creates a microenvironment predisposing to fatigue fracture (Table III).1,7,50,72,78,86-88 Junctions between the modular components are vulnerable to fretting and galvanic or pitting corrosion.33,70,89 Reciprocating movement at the interface causes fretting corrosion, which is also described as mechanically assisted crevice corrosion (MACC).90 This occurs when there is enough space at the junction to allow an aqueous solution to enter. A combination of fluid ingress and fretting initiates a repassivation reaction and ion release. Repassivation involves reaction of the freshly exposed Ti alloy surface with water to form oxide and release hydrogen ions. The negatively charged THE BONE & JOINT JOURNAL
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Table IV. Clinical reports of corrosion and/or metal ion release at modular junctions of total hip replacements Author/s Collier et al
90
Gilbert et al96
Materials at modular junction*
Findings
CoCr head + CoCr stem; CoCr head + Ti6Al4V stem; Ti6Al4V head + Ti6Al4V stem
Corrosion occurred in 25/48 mixed alloy Combination of cobalt-alloy femoral modular junction while single alloy modular head on a titanium alloy stem results in junction (n = 91) showed no corrosion galvanically accelerated crevice corrosion Two fractures at the modular The corrosion egression of surface and head–neck taper junction ingression of fluid into the prosthetic neck localised in the region of the head–neck taper junction causes failure A short and small trunnion in a patient Disassociation of femoral neck from the stem. Fretting and plastic deformation with high body mass index resulted in plastic deformation and fretting noted throughout the entire length of the corrosion that predisposed to trunnion dissociation Double tapered cone (Margron hip Even with a modern taper design and prosthesis) showed significant fretting, and corrosion-resistant materials, increased crevice corrosion of the neck–stem taper modularity can lead to fretting and crevice corrosion, metal ion generation, and particulate debris that may contribute to peri-prosthetic osteolysis and loosening Corrosion noted at neck–stem junction on Dual modular cobalt–chrome hip retrieval studies, increased blood metal ions, prostheses should be used with caution adverse tissue reactions identified on because of these concerns histology, high stress at the modular neck–stem junction found on finite element analysis 78 retrieved modular stems were assessed. Corrosion correlated to in vivo duration, Damage was common at both the patient activity and metal (vs ceramic) head–neck junction (54% showing corrosion; femoral heads, but did not correlate 88% showing fretting) and at the with head carbon content stem–sleeve junction (88% corrosion; 65% fretting) Accelerated corrosion at the Corrosion attributable to neck–stem stem–neck interface of a dual modular interface geometry, composition, neck stem (Rejuvenate, Stryker). Titanium stem length or the torque due to and CoCr neck large-diameter head
CoCr head + CoCr stem
Sporer et al84 CoCr head + CoCr neck + Ti6Al4V stem
Kop and Swartz50
CoCr head + CoCr neck + CoCr stem; Al2O3 head + CoCr neck + CoCr stem; Al2O3 head + Ti6Al4V neck + Ti6Al4V stem; CoCr head + Ti6Al4V neck + Ti6Al4V stem
Gill et al74
CoCr head + CoCr neck + CoCr stem
Huot Carlson CoCr head + Ti6Al4V stem; ceramic head + Ti6Al4V stem; Ti6Al4V stem + Ti6Al4V sleeve
et al27
Werner et al97 CoCr head + CoCr neck + Ti6Al4V stem
Conclusion
* Co, cobalt; Cr, chromium; Ti, titanium; Al, aluminium; V, vanadium; O, oxygen
chloride ions will migrate to balance positively charged hydrogen ions, producing hydrochloric acid, which can dissolve titanium or cobalt alloys.90 Mixed-alloy interfaces produce more corrosion and fretting than single-alloy couples.90-94 Combinations of alloys at a junction, e.g. Ti6Al4V and CoCrMo, can cause galvanic corrosion95 (Table IV). However, both retrieval and in vitro studies on coupling of Ti6Al4V and CoCrMo alloys have shown no increased susceptibility to corrosion,33,98 suggesting that this combination could be safe. A study on retrieved titanium modular Metha Short Hip System necks (Aesculap AG) found that fretting is often accompanied by crevice and pitting corrosion.76 Scanning electron microscopy studies have demonstrated pitting corrosion in retrieved Ti6Al4V–Ti6Al4V Profemur neck–stem junctions.91 The passive oxide layer formed in titanium alloys is destroyed by fretting and crevice corrosion, resulting in a significant reduction of strength and predisposing to fracture. The use of a CoCrMo neck was therefore recommended by Gilbert et al.91 Jauch et al99 reported similar findings at the neck–stem junction of the Metha Short Hip System with Ti6Al4V neck adapters showing significantly larger micromovements than CoCrMo neck adapters (Table IV). Contamination at the interface produced VOL. 95-B, No. 8, AUGUST 2013
significantly larger micromovements. In order to reduce the risk of fretting and possible fracture, cleaning of the taper interface before assembly and the use of a combination of different alloys at the taper interface were recommended by Jauch et al.99 Metal ion release. Many studies have linked metal ion release from hip implants with adverse peri-prosthetic tissue reactions.100,101 Most of the recent literature focuses on metal-on-metal THRs and resurfacings.102-104 However, all modular junctions have the potential to release metal ions as a result of corrosion, wear and micromovement.50,90,97,100 A study comparing MoM resurfacings with THRs having modularity at the head–neck junction found that the serum Co and Cr levels were ten and 2.6 times higher, respectively, in the THR group.105 It was therefore proposed that modular junctions play a significant role in the generation of metal ions and related adverse reactions, as well as the bearing surface itself. Ti-based components have been found to produce fewer incidences of metallosis than CoCrMo devices.78 Retrieval studies of a dual modular CoCr hip prosthesis – the Adaptor GHE/s short stem modular femoral component (Eska Implants AG, Lubeck, Germany) – demonstrated corrosion at the neck–stem taper as an important source of metal ion
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Table V. Implantation and revisions of modular neck prostheses in Australia 1999 to 2011 (adapted from the Australian Orthopaedic Association National Joint Replacement Registry Annual Report 2012; CI, confidence interval) Prosthesis*
Number implanted
Revisions (n)
Revisions/100 observed years (95% CI)
ABG II F2L M/L Kinectiv Margron Metha Profemur
223 692 1482 553 84 980
16 55 39 70 9 44
3.81 1.07 2.03 1.83 7.61 1.69
TOTAL
8300
459
1.64
* ABG II (Stryker, Kalamazoo, Michigan); F2L (Lima, Udinese, Italy); M/L Kinectiv (Zimmer, Warsaw, Indiana); Margron (Portland Orthopaedics Pty Ltd, Matraville, Australia); Metha (B Braun, Sheffield, United Kingdom ); Profemur (Wright Medical Technology, Arlington, Tennessee)
Table VI. Recall and safety notices on modular femoral stems Modular stem
Manufacturer*
Reason for recall/ safety notice
Year of recall
Metha Short Hip Modular Stem DTC Margron
Aesculap AG
Stem fracture, failure of neck adapters
2006: Ti6Al4V neck replaced with CoCr
F2L Profemur EHS Adaptor GHE/s short stem ABG II
Rejuvenate
Portland Orthopaedics Pty Ltd Lima
High revision rate due to aseptic 2008 loosening High revision rate due to aseptic 2010 loosening Wright Medical Technology Excess rates of loosening and implant 2011: Ti6Al4V neck replaced with CoCr; long fracture Ti6Al4V necks discontinued Eska Implants AG Unacceptable corrosion and metal ion release 2012: Withdrawn in Australia; still available at modular neck–stem junction in Europe Stryker Excessive fretting and/or corrosion leading 2012 to ALTR, hypersensitivity reactions and osteolysis Stryker Excessive fretting and/or corrosion leading 2012 to ALTR, hypersensitivity reactions and osteolysis
* Aesculap AG (Tuttlingen, Germany); Portland Orthopaedics Pty Ltd (Matraville, Australia); Lima (Udinese, Italy); Wright Medical Technology (Arlington, Tennessee); Eska Implants AG (Lübeck, Germany); Stryker (Kalamazoo, Michigan)
release, which resulted in high Co levels (ten times more than a monolithic femoral stem) and soft-tissue adverse reaction88 (Table IV). National Joint Registry data, recall and safety notices. The 2012 Australian Orthopaedic Association National Joint Replacement Registry Annual Report106 records that 8300 femoral stems with a modular neck were used in primary THRs between 1 September 1999 and 31 December 2011. They reported a significantly higher revision rate of 10.6% at ten years for modular neck prostheses, compared with 6.3% for fixed femoral stems. Seven exchangeable femoral neck systems had a cumulative rate of revision that was at least twice as high as that of fixed neck stems (Table V). This increase was attributed to a higher rate of loosening (3.6% vs 2.0%), dislocation (1.8% vs 1.1%) and fracture (1.4% vs 0.8%), and various prostheses were subsequently withdrawn or modified during this period (Table VI).107,108 In the United Kingdom, 6332 modular neck prostheses were implanted between 2002 and 20114 (Table VII). In the National Joint Registry for England and Wales, the performance of implants is compared using the patient time incidence rate (PTIR), defined as the number of revisions per
Table VII. Total number of modular neck prostheses implanted in the National Joint Registry for England and Wales (NJR) and Australian Orthopaedic Association National Joint Replacement Registry (AOANJRR) 2000 to 2011 Registry
Prostheses (n)
NJR
AOANJRR
6332
8300
100 patient-years. The recently withdrawn ABG II cementless (Stryker) modular-neck system had a PTIR of 4.14, and all modular neck prostheses had an overall PTIR of 0.83. This is considerably larger than the PTIR of 0.664 for all cementless stems, demonstrating that many modular neck stems are being revised much earlier than stems without modularity at the neck. Concerns exist over excessive fretting and/or corrosion leading to ALTR, hypersensitivity reactions and osteolysis. This was recently highlighted with the recall of the ABG II and Rejuvenate modular-neck prostheses (both Stryker).1 THE BONE & JOINT JOURNAL
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The United Kingdom Medicines and Healthcare Products Regulatory Agency (MHRA) has suggested regular clinical follow-up of patients with modular neck prostheses.1 In patients with local symptoms, cross-sectional imaging (metal artefact reduction sequence (MARS)-MRI or ultrasound) and the measurement of metal ion (Co and Cr) levels are recommended. If an adverse reaction to metal debris is revealed, revision to a non-modular stem may be required.
Conclusions The theoretical advantages of proximal femoral modularity include increased versatility during surgery, which facilitates accurate reconstruction of the mechanics of the hip joint, thereby increasing its durability. This could also reduce the inventory required and the cost. However, at present there is insufficient evidence to confirm these perceived benefits. The additional metal junction is vulnerable to mechanical failure, component disassociation and mechanically assisted crevice corrosion. The last of these has resulted in adverse soft-tissue reactions and the recall of modular neck designs. There is a need for further research into the prevalence and mechanism of this problem so that surgeons can decide which designs to use in the future and which patients will need close surveillance. 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 S. Hughes and first-proof edited by J. Scott.
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