- Five years after total hip arthroplasty, the proportion of shells presenting with radiolucencies or persistent gaps was higher in patients treated with a porous titanium-coated (PTC) acetabular shell than in those with a plasma-sprayed shell
- Patients with a PTC shell reported more pain and less satisfaction with the surgery, although these findings were unrelated to radiolucency
- No patient underwent revision surgery due to acetabular component loosening
In 2007, the Harris Orthopaedic Laboratory at Massachusetts General Hospital undertook a 10-year multicenter prospective study to compare a then-new cementless porous titanium-coated (PTC) acetabular shell with a proven cementless plasma-sprayed (PS) acetabular shell.
Henrik Malchau, MD, PhD, director emeritus of the Harris Orthopaedic Laboratory, and colleagues recently published five-years of data from this ongoing study in the Journal of Bone and Joint Surgery. They report an increased risk of radiolucency in the PTC shell group, but the clinical relevance of this finding is uncertain because, so far, no patient has needed revision for component loosening.
Nine hundred and seventy-six subjects, ages 20 to 75 years, who were undergoing unilateral total hip arthroplasty (THA) were enrolled between November 2007 and November 2012 at 17 centers in the United States, European Union and Mexico.
The type of acetabular shell to be used (PS or PTC) was randomly allocated by a center. Fifteen centers were assigned to perform half of their surgeries with a vitamin E-infused polyethylene (VEPE) liner and the other half used a moderately cross-linked polyethylene liner.
Of the enrolled patients, 380 had five-year follow-up radiographs as of June 16, 2016. They included 191 patients with a PTC shell and 189 with a PS shell.
The researchers defined a gap as ≥0.5 mm of radiolucent distance between the cup and acetabulum that was detected on the first postoperative radiograph (within 10 months of surgery). If a radiolucent line of the same or greater thickness was found on a later follow-up radiograph for the first time, it was defined as radiolucency.
Postoperative gaps were significantly more common with the PS shell (40% vs 24%, <.001), the research team found. However, significantly more of the PS-shell gaps disappeared within five years (96% vs 44%, <.001). This observation implies significantly better osteointegration of PS shells, the researchers explain.
At five years, 23% of the PTC shells had a radiolucency compared with only 5% of the PS shells (<.001). Regression analysis revealed a five-fold increase in the odds for radiolucency with the PTC shell (<.001), implying a significantly greater risk of bone resorption.
In fact, the researchers found that 12 patients (6%) treated with a PTC shell had radiolucent lines in all three DeLee and Charnley zones of the acetabulum.
The entire study cohort showed excellent improvement in all patient-reported outcome measures. These included both hip-specific and general health rating instruments, such as the EuroQol-5 Dimensions (EQ-5D), a numeric rating scale for average THA-related pain during the previous month, a numeric rating scale for satisfaction with the THA and the Harris hip score (HHS).
There was no significant difference between the two shell groups in the five-year HHS or EQ-5D scores. However, subjects with a PS shell reported significantly less pain and significantly greater satisfaction, compared with the PTC group. The median improvement in pain score was −5.0 (IQR, −6.3 to −3.0) in the PS group versus −4.0 (−5.5 to −3.0) in the PTC group (P=.02).
On multivariable regression analysis, no factor other than the shell material was associated with pain. The researchers expressed surprise that pain was not related to radiolucency since the PTC shell was independently associated with both increased risk of radiolucency and increased pain.
No patient underwent revision surgery due to acetabular component loosening within the study period. This makes the clinical relevance of the radiographic findings unclear, the researchers say. Hopefully, a longer-term analysis of the cohort will provide more insights into the clinical relevance of these radiographic findings and patient-related outcomes.
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