Outcomes of Metal-on-Metal THA Poorer with Modular Stems

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When metal-on-metal total hip arthroplasty (THA) fails, adverse local tissue reactions associated with metal ion release are often the cause.

The S-ROM modular stem, which has been clinically available for 32 years, is a modular stem with a porous coated proximal sleeve part. It has an additional metal-to-metal interface from which metal ions may emanate. However, little has been published about the effect of stem type on outcomes. Now, investigators at Massachusetts General Hospital have found that modular stems are significantly more likely than non-modular stems to be associated with poor radiographic and functional outcomes after metal-on-metal THA. Their findings were published in The Journal of Arthroplasty.

As part of a prospective multicenter study, Orhun K. Muratoglu, PhD, director of the Harris Orthopaedics Laboratory at Mass General, and Henrik Malchau, MD, PhD, director emeritus of the Harris Orthopaedics Lab, reviewed 539 patients who received the metal-on-metal Articular Surface Replacement THA (ASR XL), (DePuy Synthes, Warsaw, IN) unilaterally. In each case, the THA was coupled with one of three DePuy stems:

• Summit Tapered Hip System (48% of patients)
• Corail Total Hip System (35%)
• S-ROM Modular Hip System (17%)

There was no difference in the prevalence of adapter sleeves used among the stem groups. The cohort was 54% male, with a mean age of 60 years (range 23–94) and mean femoral head size of 48 mm (range 39–59). The mean time from primary surgery was 6.4 years (range 3–11).

The researchers found no significant difference between stem types in blood metal ion levels. Even so, patients with S-ROM stems were 3.8 times more likely to have osteolysis (P = .003) and 7.6 times more likely to have radiolucency (P < .001) than patients with Summit stems. The researchers speculate that these results might be related to the proximal sleeve in S-ROM stems, which creates modularity and an additional metal junction.

Four of five patient-reported outcomes were significantly worse for S-ROM stems than Summit stems. On average, S-ROM patients scored 5.1 points lower on the Harris Hip Score (0–100) (P = .023), 0.75 points higher on a visual analog scale for pain (0–10) (P = .003), 0.66 points lower on the University of California at Los Angeles activity scale (0–10) (P = .022), and 0.101 points lower on the EuroQoL 5 dimensions index (0–1) that measures quality of life (P =.001). Overall satisfaction, measured on a visual analog score, did not differ significantly between the groups.

The researchers calculated that the differences in patient-reported outcomes ranged from five to ten percent, which is clinically significant. They say that one explanation for poor function with the S-ROM might be patient selection. Before being recalled in 2010, modular S-ROM stems were often used for patients with complex anatomy, such as arthritis due to developmental dysplasia of the hip (DDH). Indeed, in this study, DDH was far more common in the S-ROM group than in the Summit or Corail groups (23%, 0.4% and 0.5%, respectively). Statistically, though, a diagnosis of DDH did not affect the association between stem type and patient-reported outcomes.

The researchers conclude that patients who have undergone large-head metal-on-metal THA with an S-ROM stem should have detailed follow-up with regular functional assessment, radiographic imaging and blood metal ion measurements. They urge caution in generalizing their results beyond the ASR XL because other hip systems have different cup designs.

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