Long Leg Radiographs vs CT Perth protocol: Mechanical alignment analysis

Mathew Cehic, Emma Jackman, Chris Wilson

Research output: Contribution to journalArticlepeer-review


Background: Long leg radiographs are commonly used to measure alignment in Total Knee Arthroplasty surgery. However, they are time consuming to perform and provide no information regarding implant rotation. The aim of this study was to compare the accuracy of measuring mechanical alignment in the coronal plane between long leg radiographs and CT Perth protocol.

Methods: Patients were recruited from our waiting list for routine TKA surgery. Pre-operative and post-operative Long Leg Radiographs and Perth protocol CT scans were performed. LLR’s were reported by 2 blinded orthopaedic surgeons and the CT scans by a blinded consultant radiologist. Intra-class correlation coefficient was compared between the scan results.

Results: Pre-operatively 132 (73 female and 59 male), meet the inclusion criteria with 120 patients completing the post op scans due to loss to follow up. The agreement between the pre-op LLD’s and CT scans was excellent with and average difference of 1.8 degrees and an ICC of 0.82. Post-op the agreement was good with an average difference in measurements of 1.58 degrees and an ICC of 0.69.

Conclusion: This study show that CT Perth Protocol achieves similar, reproducible results in determining mechanical alignment in the coronal plane when compared to LLRs. CT scans provide more information and integrate with modern robotic surgical techniques. We propose that all patients undergoing elective TKA should have a pre-operative CT for planning purposes.
Original languageEnglish
Pages (from-to)1981-1985
Number of pages5
JournalOpen Access Journal of Biomedical Science
Issue number4
Publication statusPublished - Jul 2022


  • Total Knee Arthroplasty
  • Mechanical Alignment
  • CT Perth Protocol
  • Long Leg Radiography


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