Risk of Subluxation and Impingement in vivo with
Risk of Subluxation and Impingement in vivo with Large Diameter MOM Bearings (ORS P 1789) 1, 2 Clarke IC. , 2 Burgett M. , 2 Donaldson, TK. , 3 John, A. , 4 Smith EJ. , 5 Mc. Pherson E. , 6 Savisaar C. and 6 Bowsher JG. 1 Dept. Orthop. , Loma Linda University, Loma Linda CA, 2 DARF Center, Colton CA, 3 Dept. Orthop, University of Cardiff, UK, 4 Dept. Orthop, University of Bristol, UK, 5 Los Angeles Orthopedic Institute, Los Angeles CA, and 6 FDA, Silver Springs MD Introduction: Little is known about the merits of design features in total hip arthroplasty (THA). On the one hand, a cup with sub-hemispherical profile provides greater range of motion (ROM) before impingement (Fig. 1 a: CFA =160 o). On the other hand, a cup with hemispherical profile provides better protection for the femoral head during impingement or subluxation events (Fig. 1 b: CFA=180 o). Thus parameters such as head diameter and neck thickness/symmetry interact with cup design. Such a design dilemma proves important because impingement has been the nemesis for both metal-polyethylene (MPE)[1, 6, 9, 17] and metal-onmetal (MOM) designs. [4, 8, 11, 13] In this regard, recent MOM retrieval studies revealed severe surface damage, termed ‘stripe wear’, on large diameter femoral heads. [3, 14] Stripe wear has been well described with ceramic-onceramic (COC)[7, 15] but only recently for. Such damage may be caused by (i) 3 rd body wear debris or (ii) cup rim imprinting the head, i. e. plastic deformation or (iii) some combination. [10] Fig. 1. Profiled scratch dimensions with respect to (i) raised lips, (ii) extended side Shoulders and (iii) valley depth. Fig. 1: Acetabular cups at impingement positions: (A, B) High and low profile cups impinging on 12. 5 mm THA necks (C) RSA femoral head with stripe damage shown at basal, equatorial and polar sites. Main wear (MWZ) zone (colored red) was created by pattern of habitual wear in vivo Many parameters may promote “edge wear”, including cup profile, component placement, hip-joint laxity, impingement-subluxation, microseparation and patient activities. Clinical studies have therefore used surrogate assessments such as “Arc of Cover” (ARC)[5] and “Contact Patch to Rim” (CPR)[12] with some success, i. e. radiograph assessments taken in supine position. In this report we introduce the concept of the ‘Cup Impingement Angle’ (CIA) that relates head and cup features (diameter, neck size, cup profile, etc) to the risk of impingement damage. We evaluated stripe damage on heads from THA (N=12) and resurfacing arthroplasty (RSA) (N=12) at specific sites termed “basal, equatorial, and polar" (Fig. 1 c). We collected 12 RSA retrievals to compare to 12 THA by matching vendors and MOM diameters. We hypothesized; 1) main-wear zone (MWZ) patterns (normal wear) would be similar for RSA heads and THA, 2) polar stripe angulation on RSA (adverse wear) would be greater than THA (Fig. 1 c: Qo), (3) basal stripe angulation (Bo) on RSA would be greater than with THA, and (4) 3 rd-body wear would be inconspicuous in RSA retrievals. Results: Discussion: RSA and THA heads showed MWZ areas (Figs. 1 c, 3) ranging overall 1, 085 - 3, 121 mm 2 with no statistically significant difference evident (p=0. 21). In cups the MWZ patterns ranged 1, 268 - 3, 927 mm 2, also not a statistically significant difference (p=0. 43). However MWZ areas normalized with respect to cup diameter for RSA and THA provided %Hemi-ratios 78% and 59%, respectively. This was a statistically significant difference for cups (p=0. 01). Fig. 3: Two RSA heads showed a variety of stripe damage (colored blue): A) 54 mm BHR with 1 long polar stripe (vertical) crossing 1 short polar stripe B) 45 mm ASR with 2 short polar stripes (horizontal) and a matrix of equatorial stripes. We determined that normal and adverse MOM wear patterns were similar for RSA and THA retrievals. In this study, polar stripes on both types represented unequivocal evidence that patients had routinely achieved terminal motion with sufficient force for cup rims to damage Co. Cr heads. Basal scratches in particular were clearly illustrative of 3 rd-body abrasion, with raised, ragged edges, punctuated terminuses and adjacent crater-like depressions (Figs. 4, 5. Being in the non-wear zone (Fig. 1 c), these surfaces also exhibited the highest roughness. Thus the RSA design exhibited same 3 rd-body wear damage as THA, confirming the prior observation that it was cup-to-head impingement that produced Co. Cr particulates in vivo. [10] However this calls into question whether the polar stripes were thus, (i) 3 rd-body wear by trapped metal particles, (ii) plastic deformation by the cup rim, or (iii) a combination of both? [3, 10, 16] The CIA parameter demonstrated that for small THA diameters, the impinging rim of a hemispherical cup would imprint the femoral head on the opposite hemisphere, i. e. negative CIA angulation and better head protection. As headsize increased, the CIA angulation decreased, thereby exposing the polar head area to the cup rim and with a low-profile THA cup, even more so. This may have promoted more aggressive release of metal particulates at impingement ties. Unfortunately there is no body of evidence to define such confounding issues. However it can be noted that contemporary designs with low-profile cups Discussion: have performed less well than hemispherical cups. [12 -14] All heads revealed polar stripes, often extending to MWZ boundaries multiple polar and equatorial stripes were visible on some (Fig. 3). Polar stripes had angulation (Fig. 1 c: Qo) ranging 5 -37 o and 0 -37 o with RSA and THA, respectively, with no statistically significant difference (p=0. 17). Basal stripes varied in length but rarely extended into MWZ areas. Their angulation (Bo) ranged 0 -60 o with RSA and 3 -41 o with THA. This was a statistically significant difference. Most basal scratches had conspicuous raised edges (lips) and terminated at either a smooth or punctuated terminus (Fig. 4). Basal roughness indices (Ra) proved to have the highest magnitude (Fig. 5) compared to polar and equatorial regions, but did not show a statistically significant difference between RSA and THA (p=0. 28). Methods: Visual inspections by two operators defined MWZ areas and types of stripe damage (Fig. 1 c). The bearings included Biomet (N=2 each RSA and THA), Depuy (N=5 each) and Smith & Nephew (N=5 each), totaling 22 MOM retrievals. Implants were examined by white-light interferometry (WLI: New. View 600, Zygo) and abrasion damage characterized by scratch profiles (width, lip height, valley depth). SEM imaging gave details of surface topography (SEM/EDS: MA 15, Zeiss) Graphical models were used to represent MOM designs at impingement (Figs. 1, 2). [4] ROM and impingement angles (CIA) were calculated using parameters of cup diameter Rc, neck thickness (t), and cupface angle (CFA). The RSA impingement would be complicated and undeterminable due to retention of the natural femoral-neck geometry. Also RSA cup profiles were shown to vary over a 145 -170° range (Fig. 1 a). [16] Our RSA models were therefore created utilizing ROM data from a laboratory study in which typical ROM arcs in coronal and sagittal planes averaged 78 o and 135 o respectively. [2] Thus the ‘typical’ RSA head/neck ratios, neck thicknesses and CIA values could be calculated (Fig. 2). Fig. 5: Evidence of 3 rd body abrasion damage on basal surface of THA head: A) Gray WLI image of damage zone (radial basal direction indicated) B) Corresponding WLI 3 D-imaging (blue = valley, red = peaks) C) Basal surface profile approximately 8 um deep across scratches G 1 - G 3 Acknowledgements: Fig. 4. SEM image shows multiple basal scratches approximately 100 microns wide, the majority featuring ragged, raised lips (as at ‘a’). Some scratches ended in a smooth terminus ‘b’ and others in a punctuated terminus ‘c’. References: Fig. 6. Cup impingement angles (Fig. 2: CIA) calculated for 28 -60 mm diameter THA. Here the high-profile cups are compared to the low-profile cups. With the assumed THA geometry, (12. 5 mm neck size) the CIA angulation for a hemispherical cup design (CFA=180 o) was always positioned in the negative direction. The CIA angulation for sub-hemispherical cups (CFA=160 o) was always positioned in the positive direction. With identical femoral necks this difference in CIA angulation was always 20 o. Fig. 2: Sketch of THA cup-to-neck impingement relating CIA and ROM angles to cup radius (Rc), neck thickness (t) and cup profiles, i. e. (A) sub-hemispherical cup where CFA = 160 o and (B) hemispherical cup where CFA = 180 o. DARF Center 900 E Washington St, Ste 200 Colton, CA 92324 Phone: (909) 882 -5867 ext 119 Fax: (909) 503 -1300 Email: m. burgett@darfcenter. org This Mo. M retrieval study was supported by FDA’s Critical-path Project Funding, entitled ‘‘Analysis and Validation of Wear and Corrosion Performance of Metal on Metal Hip Explants’’ (Principal Investigator ICC, FDA-1090360). Grateful thanks are also due to T. Halim of DARF Center for technical assistance. www. darfcenter. org [1] Barrack RL, Schmalzried TP. J Bone Joint Surg Am, 84 -A (2002) 1218. 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