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Perth CT protocol shows impact of tibial component rotation on TKR outcomes
A single radiographer applied the Perth CT protocol, which considers seven alignment characteristics, to post-TKR 2.5-mm slices of scans of 346 patients.
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In a detailed clinical study using the Perth CT protocol, a tool for assessing the alignment of knee prostheses, investigators were unable to correlate femoral component alignment in primary total knee replacement with pain, range of motion or Knee Society Scores at 1 year postoperatively. However, they did find tibial component internal rotation significantly influenced range of motion and Knee Society Scores when it exceeded 7° to 9°.
Furthermore, they observed a trend towards greater pain levels, all of which has implications for surgeons doing total knee replacement (TKR).
In this study, which is scheduled to be presented at the EFORT Congress in Istanbul, Karen E. Sloan, MS, a research associate at Royal Perth Hospital, in Perth, Western Australia, Australia, and colleagues first tested the clinical reliability of the Perth CT protocol. They then used the protocol in the second part of the study to determine the relationship between outcome and component rotation following TKR in 346 patients with osteoarthritis.
“The thing we want to point out is that although we did find that there does seem to be a link with internal rotation (IR) of the tibial component, in particular, and a poorer outcome, there were still patients who had IR of the tibia who had a perfectly fine outcome. So outcome is multifactorial for knee replacement,” Sloan told Orthopaedics Today Europe.
CT slices
One radiographer measured the CT scans taken at 6 months postoperatively of the TKRs of all the patients with a TKR in the study, Sloan said. The Perth CT protocol involves reviewing seven alignment characteristics of the implant using bony landmarks from the acetabulum to the talus.
Image: Sloan KE
“We found that IR of the tibial component might be associated with a poorer outcome and this was statistically significant at a threshold around 7° to 9°,” Sloan said.
She noted, however, that the statistical significance does not necessarily mean there are differences in outcome that are clinically significant or meaningful.
“People are certainly starting to look at IR of the tibial component as something that might be associated with a poorer outcome,” Sloan said. “You have to be slightly careful about whether that is really a poor outcome or not. Our differences between groups were small. However, there certainly appears to be a relationship between IR of the tibial component and outcome in the measurements that we looked at.”
Real-life setting
TKRs assessed in this study were performed by multiple surgeons in a large hospital setting and involved different implants. These were mainly the Triathlon Total Knee and Duracon Total Knee System (Stryker Orthopaedics; Mahwah, N.J., USA) and the Profix Total Knee System (Smith & Nephew; Memphis, Tenn., USA), according to Sloan.
“They tended to be pressfit femoral and cemented tibial components,” Sloan said, noting some of the patients’ prostheses were implanted using navigation, but mobile-bearing prostheses and those implanted using a gap-balancing technique were excluded.
Sloan and colleagues collected the patients’ Knee Society Score (KSS) pain subcomponent measurements and range of motion at 1 year postoperatively and assessed their knee alignment using the CT protocol. They initially divided patients into two groups: those with pain and those who were pain-free. No significant differences were found in femoral or tibial component rotation between the two groups; however there was a tendency for the painful group to have greater tibial IR.
The second assessment involved placing patients in the two pain groups into subgroups based on IR of, firstly, the femoral component, and then the tibial component to determine if there was a threshold at which outcome is affected.
The investigators found tibial component IR results were significantly different for KSS (P=0.001) and range of motion (P=0.000) outcomes beyond 7° to 9°. Using this upper threshold, a larger percentage of patients in the pain group had tibial IR great than 9°, compared to those with no pain, which while a trend, was not significant.
Sloan said one of her study’s weaknesses was its use of the KSS score as a measure of outcome.
“It may not be sensitive enough and we would like to maybe look at more sensitive measures to delve further,” she said and noted another limitation was the fact that the number of poorly aligned and poorly performing implants included in the study was small. – by Susan M. Rapp
References:
Chauhan SK. J Bone Joint Surg Br. 2004;86(6):818-823.
Sloan KE. Paper #13-1537. Scheduled to be presented June 7 at the EFORT Congress; June 5-8, 2013. Istanbul.
For more information:
Karen E. Sloan, MS, can be reached at Joint Replacement Assessment Clinic, Shenton Park Campus, Royal Perth Hospital, 6 Selby St., Shenton Park, Perth, Western Australia, Australia; email: karen.sloan@health.wa.gov.au.
Disclosure: Sloan has no relevant financial disclosures.