Activity and socket wear in the Charnley low-friction arthroplasty

The Journal

Activity

and Socket Low-friction

of Arthroplasty

Vol. 9 No. 4 1994

Wear in the Charnley Arthroplasty

Julian A. Feller, FRACS,* Peter R. Kay, FRCS,t John P. Hodgkinson, FRCS,fand B. Michael Wroblewski, FRCS+

Abstract: A clinical study was undertaken to assess the influence of patient-related factors on wear of the socket in Charnley low-friction arthroplasty. One hundred nint arthroplastiesin 79 patientswere reviewed at an averageof 10.3 years. A new method of activity assessment wasdesignedand usedto estimatethe distancewalked by each patient. This method was validated by a pedometerthat recordedthe time taken by each patient to walk 20 m. Activity level was related to the amount of movement at the level of the prosthetic articulation by calculating the sliding distanceof a point at the center of the surfaceof the head of the femoral component. Wear was found to correlate with patient activity, but not with the physical characteristicsof the patient or the time sinceoperation. Key words: Charnley low-friction, socketwear, activity level.

Wear of the acetabular component of the Charnley low-friction arthroplasty (LFA) can be assessed radiographically, as described by Charnley and Halley.’ Earlier radiographic methods of wear measurement’ have been criticized.3 However, more recent techniques have been validated in vitro by using a model” and in vivo by correlating radiographic wear with direct measurements of casts of retrieved components.5 The factors influencing wear of the acetabular component are not yet fully understood. The molecular weight,‘,’ elastic modulus,8 and thickness8,9 of the polyethylene have all been shown to affect the wear characteristics of the acetabular component, as has the conformity of the prosthetic articulation.8,1” Contact stresses above certain levels are important and may be the common pathway for the effect of a number of variables.2,8,1’ Bone-cement particles, particularly clusters of the constituents added to provide radiographic contrast, have also been shown to be of importance.i2-15

Of the clinical factors investigated, the weight of the patient has consistently been shown not to be of relevance, 1.2.4.5.16,17 Wroblewski” hypothesized that the product of weight and time since operation could prove to be a significant factor. Griffith et aL4 demonstrated that there was greater wear in men, while Rimnac et al.” found that the height of the patient was not significant at all. There is conflicting evidence regarding both the age of the patient4,16 and the time since operation.‘.16 Similarly, while Charnley and Halley’ concluded that patient activity did not influence cup wear, Griffith et a1.4 found that there was a preponderance of active patients in the group with increased wear. Assessment of activity levels in these studies was based on a relatively simple scheme of qualitatively grading a patient’s walking ability, taking into account the influence of other joint involvement and systemic disease.” Wroblewski I6 observed that the amount of acetabular wear had not been related to the distances walked by different patients. The distance walked can be theoretically cxtrapolated to the total movement at the prosthetic articulation. This can be expressed quantitatively as the sliding distance-a value representing the total sum of

From *Austin Hospital, Melbourne, Australia, tAncoats Hospital, Manchester, and #The Centre for Hip Surgery, Wrightington Hospital, Wigan, Lancashire. United Kingdom. Reprint requests: Mr. J. A. Feller, l/210 Burgundy Street, Heidelberg 3084, Australia.

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culated from the maximum variation in the distance between the femoral head and wire marker on the most recent radiograph and a radiograph taken prior to hospital discharge. The small effect of creep is therefore included in this measurement.‘z,“’ Patient Activity

:

:

: 8’

:

:

L/ : 0’ : : 8’ : : : 8’ : : : : : :~

1. The conceptof slidingdistance.SD, slidingdistance for a singlestep; L, leg length; D, length of a singlestep (half a stride length). Fig.

relative excursion between the surfaces of the prosthetic femoral head and acetabulum (Fig. 1). The purpose of this study is to assessthe long-term effects of patient activity, the physical characteristics of a patient, and the time since operation on wear of the acetabular component of the Charnley LFA.

Materials

and Methods

One hundred nine Charnley LFAs in 79 patients attending for routine, long-term (> 8 years) followup examination were evaluated. All prostheses had been implanted as primary procedures at The Centre for Hip Surgery, Wrightington Hospital. Wear of the Acetabular

Component

Wear was assessedradiographically, as described by Charnley and Halley.’ In this technique, after correction for magnification, the amount of wear is cal-

Patient activity was assessedby estimating the distance walked per week by each patient and by calculating the sliding distance for each arthroplasty based on this estimated distance. A questionnaire was designed to estimate the distance walked by a patient based on a typical week’s activity since operation. It was administered in the form of a loosely structured interview, which lasted between 45 and 60 minutes. The questionnaire served as a checklist of specific information to be obtained. The topics covered were type of accommodation, occupation, percentage of time spent walking, sitting, standing, and carrying weight, the number of stairs negotiated, transport to and from work, shopping, hobbies, the use of walking aids, and the involvement of other joints. Based on these data, the patients were assigned to one of five activity grades, to each of which a mileage range had previously been allocated (Table 1) To validate these estimates, 2 1 patients used a pedometer for a week. The pedometer counted the number of steps taken by one limb. By calculating the average step length of a patient from the number of stepstaken to cover 20 m at the follow-up evaluation, the pedometer reading was used to estimate the actual mileage walked. This was compared with the mileage estimated from their assignedactivity grade. In order to provide further validation of the activity grading system from an alternative perspective, the assigned activity grade for each patient was correlated with the time taken to walk 20 m at the followup evaluation. Sliding distance per year was calculated according to the formula: d = 3.25.n.sin- ’

(‘1 where d = n = s= 1= x =

sliding distance per year (m), number of steps to walk 20 m, stride length (cm), leg length (cm), miles walked per week; this distance was taken as the average value of the activity grade to which the patient had been assigned (Table 1).

Socket

Table 1. Relationship Between Activity Grade and Miles Walked Per Week Miles Walked

Activity -

Grade

Per Week O-l 2-5 6-10 1 l-20 21-40

1 2 3 4 5

Assigned Mean Value 0.5 3.5 8.0 15.5 30.3

Physical Characteristics The following physical characteristics were recorded for each patient: sex, age, weight, height, diagnosis (primary osteoarthritis, secondary osteoarthritis, or rheumatoid arthritis), side, leg length, and stride length. Leg length was measured from the tip of the greater trochanter to the floor. Stride length was calculated from the number of steps required to walk 20 m, one stride being from heel strike to heel strike of the same limb, that is, two steps. Each characteristic was correlated with acetabular component wear. Time The time since operation was recorded for each arthroplasty. To investigate the effect of time when combined with other variables, sliding distance and patient weight were each multiplied by the time since operation and correlated with wear. Statistical

Analyses

Statistical analyses were performed using Pearson’s product moment correlation coefficient and Spearman’s rank correlation coefficient. The critical value of P was set at .05.

Results Wear The mean wear of the acetabular component measured was 1.1 mm (range, 0.0-6.6 mm). Patient Activity The mean assignedactivity grade was 2.98 (range, l-5). For those patients who used a pedometer, there was a strong and statistically significant correlation between the mileage estimated from the activity grade and the measured mileage (r = ~303, P < .OO1) There was also a statistically significant inverse correlation between the assigned activity grade and the time to walk 20 m (r = -.609, P < .02). The

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Table 2. Details of Recorded Physical Characteristics Characteristic Sex(M:F)

40:60 49:51 72:18:10 Mean ___-

Side (L:R) Diagnosis ( 1°0A:2”OA:RA)

Age(years)

56 77 I66 68 90

Weight (kg) Height (cm) Stride length (cm) Leg length (cm) OA, osteoarthritis;

RA, rheumatoid

Range 20-75 38-146 145-183 24- 129 76- 106

arthritis.

sliding distance per year averaged 20,638 m/y (range, 984-67,408 m/y) and correlated strongly and significantly with activity grade (r = .972, P < .OO 1). There was a statistically significant correlation between activity grade and acetabular component wear (r = .4533, P < .OOl), and between sliding distance per year and acetabular component wear ( r = .42 14, P < .OOl). Physical Characteristics The mean and range of each of the physical characteristics recorded are shown in Table 2. There was no statistically significant correlation between any of the characteristics and acetabular component wear. Time Since Operation The mean time since operation was 10.3 years (range, 8.3-16.3 years), and there was no statistically significant correlation between time since operation and acetabular component wear. The product of patient weight and time since operation did not correlate with acetabular component wear as well.

Discussion This study demonstrates that, of a variety of patient-related factors, only measurements of activity correlate significantly with acetabular component wear. While other variables undoubtedly contribute to component wear, the effect of any such factors will be magnified by a high level of patient activity. To our knowledge, the level of activity of patients who have undergone total hip arthroplasty has not previously been quantified in terms of distance walked. Our method of assessingpatient activity and assigning an activity grade has proven to be reliable. This was shown by the strong correlation between the distance walked as estimated from the assigned activity grade and the distance walked as measured

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with a pedometer. The method was further supported by the finding of a significant inverse correlation of the activity grade with the time taken to walk 20 m at the follow-up evaluation. Our system for rating activity is more likely to reflect the level of a patient’s activity at the follow-up evaluation than during the early postoperative years. Due to increasing age, involvement of other joints, intercurrent illness, or pain related to loosening of the prosthesis, the level of a patient’s activity may decline. However, at the follow-up evaluation, none of the patients in this series were scheduled for revision surgery. We believe that this system is the most comprehensive method reported to date. Similarly, it is worth noting that we used the total wear distance for our correlations rather than the wear rate, which may increase with time. While it may be a simplification not to normalize the rate of wear to time, we believe that this was justified by our finding that the time since operation did not correlate with wear. The concept of sliding distance was an attempt to achieve a more accurate representation of the amount of movement occurring in the prosthetic hip joint. The use of sliding distance, however, did not improve the correlation between activity and wear. This is almost certainly because patient activity is the most significant variable in the calculation of sliding distance and far outweighted the variables of leg length and stride length. In our series, stride length did not exceed leg length in any individual. WC have calculated that a variation in stride length from 10 cm up to leg length can only cause a variation in sliding distance at the prosthetic articulation of less than 5%. Based on our findings, we do not believe that the calculation of sliding distance need be included in future studies evaluating the role of activity following total hip arthroplasty. we did not find Like previous authors, 1.2,4.5,1~~,17 any correlation between the weight of the patient and acetabular component wear. Nor did the product of patient weight and time since operation correlate significantly with wear. This is not surprising given the lack of influence of either time or weight alone. Although patient weight may be important in total knee arthroplasty, this apparently is not the case with the Charnley LFA or with any total hip arthroplasty. This is presumably because the contact pressures at the metal-polyethylene interface do not reach a critical level in total hip arthroplasty because of greater congruency between the components and a correspondingly larger contact surface area than occurs in total knee arthroplasty. 7,8 Measurements for total hip arthroplasty occur on the flatter part of the contact pressure/wear curve.’ ’

Despite findings of other authors,‘,‘” we did not find any correlation between wear and sex or age of the patient. As previously noted, evidence regarding the effect of age is conflicting.“,‘” We confirmed the observation that the height of the patient is not a significant factor,17 despite its theoretic influence on leg length and, therefore, sliding distance. Our study demonstrated that wear was significantly related to patient activity but not to other patient-related factors. In light of increasing evidence in the literature suggesting wear debris as a cause of loosening, ‘h.20,2 ’ there may well be a causal relationship between patient activity and loosening. This has obvious implications regarding the advice one gives to patients.

References 1. Charnley J, Halley DK: Rate of wear in total hip replacement. Clin Orthop 112: 170, 1975 2. Charnley J, Cupic Z: The nine and ten year results of the low-friction arthroplasty of the hip. Clin Orthop 95:9, 1973 3. Clarke IC, Black I<, Rennic C, Amstutz HC: CJ~ wear in total hip arthroplasties be assessed from radiographs? Clin Orthop 12 1: 126, 1976 4. Griffith MJ, Seidenstein MK, Williams D, Charnley J: Socket wear in the Charnley low friction arthroplasty

of the hip. Clin Orthop 137:37, 1978 5. Wroblewski BM: Direction and rate of socket wear in Charnley low friction arthroplasty. J Bone Joint Surg 67-B:757, 1985 6. Rose RM, Nusbaum HK, Schneider H et al: On the true wear rate of ultra high-molecular-weight polyethylene in the total hip prosthesis. J Bone Joint Surg 62A:537, 1980 7. Rose RM, Radin LL: Wear of polyethylene in the total hip prosthesis. Clin Orthop 170: 107, 1982 8. Bartel DL. Bicknell MS, Wright TM: The effect of corlformity, thickness, and material on stresses in ultrahigh molecular weight components for total joint replacement. J Bone Joint Surg 68A: 1041, 1986 9. Collier JP, Mayor MB, Surprenant VA et al: The biomechanical problems of polyethylene as a bearing surface. Clin Orthop 261: 107, 1990 10. Wright TM, Rimnac CM, Faris PM, Bansal M: Analysis of surface damage in retrieved carbon fiber-reinforced and plain polyethylene tibia1 components from posterior stabilized total knee replacements. J Bone Joint Surg 7OA:1312, 1988 11. Rostoker W, Galante JO: Contact pressure Jependence of wear rates of ultra high molecular weight polyethylene. J Biomed Mater Res 13:957, 1979 12. Atkinson JR, Dowson D, Isaac GH, Wroblewski BM: Laboratory wear tests and clinical observations of the penetration of femoral heads into acetabular cups in total replacement hip joints. Part 2. A macroscopic

Socket

study of the surfacesof Charnley polyethylene acetabular sockets.Wear 104:217, 1985 13. IsaacGH, Atkinson JR, Dawson D, Wroblewski BM: The role of cement in the long term performanceand prematurefailure of Charnley low friction arthroplasties. Eng Med 15:19, 1986 14. IsaacGH. Atkinson JR, Dawson D, Wroblewski BM: The causesof femoral head roughening in explanted Charnley hip prostheses.Eng Med 16:167, 1987 15. RostokerW, GalanteJO: The appearancesof wear on polyethylene: a comparison of in vivo and in vitro wear surfaces.J Biomed Mater Res 12:317, 1978 16. IsaacGH, Wroblewski BM, Atkinson JR, Dawson D: A tribological study of retrieved hip prostheses.Clin Orthop 276: I 15, 1992

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17. RimnacCM, Wilson, FichsMD, Wright TM: Acetabular cup wear in total hip arthroplasty. Orthop Clin North Am 19~631,1988 18. Wroblewski BM: Revision surgery in total hip arthroplasty. Springer-Verlag,London, 1990 19. Charnley J: The long-term results of low-friction arthroplasty of the hip performed asa primary intervention. J Bone Joint Surg 54B:61, 1972 20. Howie DW, Vernon-RobertsB, OakeshottR, Manthey B: A rat model of resorption of bone at the cementbone interface in the presenceof polyethylene wear particles.J Bone Joint Surg 70A:257, 1988 2 1. Mirra JM, Marder RA, Amstutz HC: The pathology of failed total joint replacement. Clin Orthop 170:175, 1982