Has the threshold for revision surgery for adverse reactions to metal debris changed in metal-on-metal hip arthroplasty patients? A cohort study of 239 patients using an adapted risk-stratification algorithm

Figures & data

Table 1. Adaptation of the risk stratification of metal-on-metal hip arthroplasty patients originally proposed by Kwon et al. (2014)

Pre-revision factor	Low risk	Moderate risk	High risk
Implant	– Non-recalled hip resurfacing in men under 50 years with OA	– All other non-recalled hip resurfacing implants	– Large-diameter (≥ 36 mm) modular THA
			– Any recalled implants
Radiographic	– Optimal acetabular component position ^a	– Optimal acetabular component position ^a	– Suboptimal acetabular component position ^a
	– No implant osteolysis or loosening	– No implant osteolysis or loosening	– Any implant osteolysis and/or loosening
Blood metal ions ^b	– Both under 3 ppb	– Either or both between 3 and 10 ppb	– Either or both above 10 ppb
Cross-sectional imaging	– Within normal limits	– ARMD without muscle/bone involvement	– ARMD with muscle/bone involvement
		– Simple cystic lesions or small cystic lesions without thick walls	– Solid or mixed lesions
			– Cystic lesions with thick walls

ARMD = adverse reactions to metal debris; OA = osteoarthritis; ppb = parts per billion; THA = total hip arthroplasty.

^a Optimal position defined in methods section.

^b Chromium and cobalt

Table 2. Demographics of metal-on-metal hip arthroplasty patients undergoing revision surgery. Values are frequency (percentage) unless otherwise specified

Pre-revision details	Cohort revised (n = 239)
Mean age at revision, years (SD)	60 (11)
Female sex (%)	168 (70)
Bilateral MoM hips any (%)	93 (39)
Bilateral MoM hips revised for ARMD (%)	35 (15)
Mean time to revision in years for ARMD (SD)	7 (3)
Primary and revision centre same	182 (76)
Primary implant type
Hip resurfacing	150 (63)
Total hip arthroplasty	89 (37)
Primary implant design
BHR	104 (44)
Other (THR or HR)	51 (21)
Conserve HR	29 (12)
Corail-Pinnacle THR	29 (12)
Synergy BHR THR	26 (11)
Primary implant head size
<46	80 (42)
46 mm	60 (31)
>46	52 (27)
Symptoms
Local symptoms	221 (93)
Systemic symptoms	2 (0.8)
Blood metal ions:
Median cobalt, µg/l (IQR)	1.9 (0.7–8.0)
Median chromium, µg/l (IQR)	3.5 (1.6–8.3)
Radiographs
Mean cup inclination in degrees (SD)	49 (11)
Mean cup version in degrees (SD)	19 (10)
Cup malposition	135 (57)
Stem/head malposition	3 (1)
Loose cup	9 (4)
Loose stem	11 (5)
Lysis cup	101 (42)
Lysis stem	42 (18)
Neck thinning	34 (14)
Impingement	1 (0.4)
Heterotopic ossification	20 (8)
Any cross-sectional imaging
Any abnormality (% of those with imaging)	202 (85)
Pseudotumors (PT)
PT numbers	163 (68)
PT consistency (% of all PT)
Cystic	71 (44)
Mixed	83 (52)
Solid	7 (4)
PT location (% of all PT)
Anterior ± lateral	64 (40)
Posterior ± lateral	48 (30)
Anterior + posterior ± lateral	29 (18)
Other	20 (12)
Median PT volume, cm^3 (IQR)	45 (13–130)
Other image abnormalities
Effusion	44 (18)
Muscle atrophy/damage	17 (7)
Tendon abnormality/damage	13 (5)
Bursal distension/thickening	24 (10)

BHR = Birmingham Hip Resurfacing; HR = hip resurfacing; IQR = interquartile range; PT = pseudotumor; SD = standard deviation; THA = total hip arthroplasty.

Note that micrograms per litre (µg/l) and parts per billion (ppb) are equivalent units of measure (see Table 1).

Table 3. Adapted risk-stratification group in relation to year of revision surgery. Values are frequency (percentage)

Risk-stratification group		Revised before 2012	Revised 2012 onwards
	Overall	(n = 58; 24%)	(n = 181; 76%)	p-value
Implant				0.001
Low	11 (5)	3 (5)	8 (4)
Moderate	138 (58)	45 (78)	93 (51)
High	90 (38)	10 (17)	80 (44)
Radiographic				0.01
Low/moderate	46 (19)	18 (31)	28 (16)
High	193 (81)	40 (69)	153 (85)
Blood metal ions				0.001
Low	86 (36)	11 (19)	75 (41)
Moderate	92 (39)	23 (40)	69 (38)
High	61 (25)	24 (41)	37 (20)
Cross-sectional imaging				0.4
Low	43 (18)	7 (12)	36 (20)
Moderate	106 (44)	26 (45)	80 (44)
High	90 (38)	25 (43)	65 (36)

Kwon Y M, Lombardi A V, Jacobs J J, Fehring T K, Lewis C G, Cabanela M E. Risk stratification algorithm for management of patients with metal-on-metal hip arthroplasty: consensus statement of the American Association of Hip and Knee Surgeons, the American Academy of Orthopaedic Surgeons, and the Hip Society. J Bone Joint Surg Am 2014; 96 (1): e4.

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