 HIP

The use of a lipped acetabular liner decreases the rate of revision for instability after total hip replacement P. J. Insull, H. Cobbett, C. M. Frampton, J. T. Munro From Auckland City Hospital, Auckland, New Zealand

A STUDY USING DATA FROM THE NEW ZEALAND JOINT REGISTRY We compared the rate of revision for instability after total hip replacement (THR) when lipped and non-lipped acetabular liners were used. We hypothesised that the use of a lipped liner in a modular uncemented acetabular component reduces the risk of revision for instability after primary THR. Using data from the New Zealand Joint Registry, we found that the use of a lipped liner was associated with a significantly decreased rate of revision for instability and for all other indications. Adjusting for the size of the femoral head, the surgical approach and the age and gender of the patient, this difference remained strongly significant (p < 0.001). We conclude that evidence from the New Zealand registry suggests that the use of lipped liners with modular uncemented acetabular components is associated with a decreased rate of revision for instability after primary THR. Cite this article: Bone Joint J 2014;96-B:884–8.

 P. J. Insull, BHB MBChB, Orthopaedic Registrar Auckland City Hospital, Department of Orthopaedics, Park Road, Grafton, New Zealand.  H. Cobbett, BSc, MBChB, Orthopaedic Registrar Northshore Hospital, Takapuna, Auckland, New Zealand.  C. M. Frampton, BSc, PhD, Associate Professor, Biostatistician University of Otago, Christchurch, New Zealand.  J. T. Munro, BHB, MBChB, FRACS, Orthopaedic Consultant, Senior Lecturer Auckland City Hospital, University of Auckland, Grafton, Auckland, New Zealand. Correspondence should be sent to Dr P. J. Insull; e-mail: [email protected] ©2014 The British Editorial Society of Bone & Joint Surgery doi:10.1302/0301-620X.96B7. 33658 $2.00 Bone Joint J 2014;96-B:884–8. Received 13 December 2013; Accepted after revision 4 April 2014

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Instability after total hip replacement (THR) remains a problem,1 and is recorded as the indication for revision THR in 29% of cases in the New Zealand Joint Registry (NZJR),2 21.3% in the Australian Joint Registry3 and for between 23% and 34% in the Scandinavian registries with 49% of revisions for dislocation occurring within three years of the primary THR in the Swedish Hip Arthroplasty Register.4 Non-lipped liners have the same depth around their entire circumference. Lipped liners were designed by Charnley in the early 1970s to enhance posterior stability.5 They have an augmented rim, which is orientated in the direction of potential dislocation to prevent displacement of the femoral head (Fig. 1). For example, one can place the elevated rim posteriorly and superiorly for THRs using the posterior approach to reduce posterior dislocation, or anteriorly and superiorly for anterior or lateral approaches. Theoretically, dislocation is prevented by decreasing the arc of movement of the components.6,7 While some authors have reported a reduced rate of dislocation following the use of lipped liners,6-8 there remains some anxiety that impingement of the femoral component against the lip may cause instability and loosening by increased polyethylene loading and

wear (Fig. 2).8-14 Some authors have suggested that this may not be the case.9,15 Others hypothesise that impingement may act as a lever for dislocation in the direction opposite to the lip16,17; however, the significance of this has been questioned.5 Lipped options are not available for metal or ceramic acetabular components. The limitation of movement caused by the use of a lipped liner18 may restrict activity which could be particularly relevant in younger patients. However, we are not aware of any evidence in the literature investigating this concern. Many other factors are known to influence the risk of dislocation after THR. This risk is reduced when a higher volume arthroplasty surgeon is the primary operator.19 Accurate alignment, offset and leg length is critical for stability after THR.20,21 The use of larger bearing diameters has been shown to reduce the rate of dislocation significantly, theoretically due to an increase in head-neck offset and jump-distance.22,23 Patient factors, such as compliance, cognitive disability, obesity, neurological or soft-tissue deficiency and previous surgery are also important predictors of dislocation.24,25 To date, no large studies have looked at the relationship between the use of a lipped liner in THE BONE & JOINT JOURNAL

THE USE OF A LIPPED ACETABULAR LINER DECREASES THE RATE OF REVISION FOR INSTABILITY AFTER TOTAL HIP REPLACEMENT

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x

Fig. 1 The augmented lip increases stability by increasing the displacement required for the prosthetic head to dislocate.

Fig. 2 The augmented lip may act as a lever for dislocation in the opposite direction to which it is placed.

THR and the rate of revision for instability. We suspected that the use of a lipped liner in modular uncemented acetabular components would significantly reduce the risk of revision of THR for instability compared with non-lipped liners.

Materials and Methods Using the New Zealand Joint Registry, we identified all lipped and non-lipped polyethylene liners for three of the most commonly used modular uncemented acetabular systems.2 Survival data and cause for revision were extracted for the Pinnacle (DePuy, Warsaw, Indiana), Trident (Stryker, Manwah, New Jersey) and Trilogy (Zimmer, Warsaw, Indiana) systems. From January 1999 to December 2012, the NZJR, included 84 430 THRs in total. Of the total 7218 THRs performed in New Zealand in 2011, 2122 (29%) of these VOL. 96-B, No. 7, JULY 2014

used one of the three acetabular systems included in our analysis. Ceramic and metal liners were excluded as they do not have lipped options. Surgical indications. Cox regression analyses were performed to determine the rates of revision for both lipped and nonlipped liners and to build a predictive model. These were used to calculate hazard ratios for revision for instability based on the type of modular liner used. This was adjusted for the size of the femoral head, the surgical approach and the age and gender of the patient to control for possible bias. SPSS version 19.0 (IBM statistics, Armonk, New York) was used for our statistical analysis and a p-value of < 0.05 was considered statistically significant.

Results A total of 8023 components with lipped liners and 4088 with non-lipped liners were identified. A total of 88 THRs

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Table I. Comparative rates of revision total hip replacement for instability Summary component years

No. revisions

Rate/100 component-years

Lower 95% Confidence interval

Upper 95% Confidence interval

Lipped Non-lipped

8028 4088

26857.7 14650.5

43 45

0.16 0.31

0.12 0.22

0.22 0.41

Total

12116

41508.3

88

0.21

0.17

0.26

Proportion free of instability revision

n

1.00 0.99 0.98 0.97 Lipped Non lipped

0.96 0.95 0.94 0.93 0.92 0.91 0.90

0 1 2 3 4 5 6 7 8 9 10 11 12 Years since operation Fig. 3 Graph showing proportion revised by year since operation. Table II. Relative risk of revision for instability when using non-lipped acetabular liners

Unadjusted Adjusted for age, gender, head size and approach

Hazard ratio (Unlipped/lipped)

Lower 95.0% Confidence interval

Upper 95.0% Confidence interval

p-value

1.996 2.433

1.314 1.587

3.030 3.731

0.001 < 0.001

were revised for instability, 43 with lipped liners and 45 with non-lipped liners (Table I). This corresponds to a rate of revision for instability of 0.16 per 100 component years for lipped liners compared with 0.31 for non-lipped liners (p = 0.001) (Fig. 3). When adjusted for size of femoral head, the surgical approach and the age and gender of the patient, the cox regression analysis showed that THRs with non-lipped liners were 2.4 times more likely to undergo revision for instability than those with lipped liners (p < 0.001) (Table II). There was no significant difference in the mean age of the patient in the two groups. Combining the lipped and non-lipped liners, there was no significant difference in the rate of revision for instability between liners of 32 mm and 36 mm diameter (p = 1.0). There was also no significant difference in the rate of revision between THRs with lipped liners and a 32 mm diameter femoral head and lipped liners with a 36 mm diameter femoral head (p = 1). The same comparison for THRs with nonlipped liners also found no significant difference (p = 0.6)

The rate of revision for instability for THRs with a lipped liner is similar to that in metal-on-metal THRs for instability (0.12 per 100 component years, 95% Confidence Interval (CI) 0.09 to 0.16) and ceramic-on-ceramic (0.14 per 100 component years, 95% CI 0.10 to 0.18) THRs recorded in the registry. The rate of revision for instability in non-lipped liners was significantly higher (0.31 per 100 component years, 95% CI 0.22 to 0.41, p = 0.001). The overall rate of revision for all indications in the lipped liner group was also found to be significantly lower than that for non-lipped liners, at 0.62 per 100 component years compared with 0.76 per 100 component years (p = 0.046).

Discussion We sought to determine comparative rates of instability after THR requiring revision with the use of lipped and non-lipped polyethylene liners in modular uncemented acetabular components. We found a significantly lower rate of revision for instability when a lipped liner was used. We have shown that this effect is still strongly significant after THE BONE & JOINT JOURNAL

THE USE OF A LIPPED ACETABULAR LINER DECREASES THE RATE OF REVISION FOR INSTABILITY AFTER TOTAL HIP REPLACEMENT

statistical adjustment is made for the size of the femoral head, the surgical approach and the age and gender of the patient. We also found that the overall rate of revision was lower when a lipped liner is used. A limitation of our study is that the NZJR does not capture rates of dislocation after THR. Therefore we are unable to compare our results directly with previous studies, such as that of Cobb et al.6 Patients with unstable THRs undergoing revision may have had other indications recorded on the NZJR form at the time of revision, such as infection or aseptic loosening rather than instability. This may give a falsely low impression of the rates of revision for instability in such cases. However, it is unlikely that this would favour one type of liner over the other and bias our results significantly. Some authors have suggested that lipped liners may induce late dislocation as a result of impingement.26,27 Although we looked at the rates of revision up to 12 years post-operatively, this may not have been enough to show this phenomenon. To our knowledge, many surgeons use a 10º lipped liner; however, as our data did not specifically look at the size of the lip, this may have influenced our results. The strength of our analysis lies in the inclusion of a large number of THRs, using acetabular systems implanted in high volumes in New Zealand. The NZJR includes data from more than 98% for all joint replacements, and is therefore extremely robust.28 Previous authors have raised other concerns about the use of lipped liners, such as their propensity to crack, especially if the acetabular component is introduced with a steep abduction angle.29 To our knowledge, no large studies have demonstrated a relationship between the abduction angle of the acetabular component and damage to the liner, and further research is needed to investigate this technical issue. It is reassuring that we found the overall rate of revision for THRs where a lipped liner was used, to be significantly lower than where a non-lipped liner was used. It may simply be that the reduction in the rate of revision for instability is so great that the effect of other indications for revision is dampened. The follow-up available in the NZJR may not yet be long enough to capture a possible late increase in the rates of revision for other causes, which could be increased by the presence of the lip. We are unaware of any other registry analysis or large clinical series evaluating the influence of modular lipped acetabular liners on the long term rates of revision for instability or other causes. Despite some limitations in this study, based on these results we would cautiously suggest that the use of a lipped liner, when such an option exists, may decrease the risk of revision for instability. We have confirmed that the use of a lipped liner is associated with a significantly decreased risk of revision for instability compared to the use of a non-lipped liner with an uncemented modular acetabular component as recorded in the NZJR. This decreased risk remains strongly statistically VOL. 96-B, No. 7, JULY 2014

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significant when the size of the femoral head, the surgical approach and the age and gender of the patient are adjusted for and when all indications for revision are included. The authors would like to thank the New Zealand Joint Registry for their assistance in this study. 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 D. J. Johnstone and first proof edited by J. Scott.

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