Weight-Bearing CT and Conventional Periprosthetic Distal Knee Fractures

Total knee arthroplasty is the most cost-effective and successful treatment for knee joints with end-stage osteoarthritis, with more than 58,000 TKA surgeries in Canada in 2021-2022.1 The prevalence of TKA surgeries is increasing per year and is projected to rise due to an ageing population and obesity problems.1 In addition to the primary surgery, more than 4,000 Canadians require revision TKA surgery per year; the TKA revision burden is estimated to reach $13 billion by 2030 as a consequence of a substantial increase of 149% in primary surgeries by 2030 in the United States.1-3 Revision surgery is more invasive than primary surgery and poses the risk of increasing patient mortality rates, especially for older adults.3 Thus, it is imperative for surgeons to make an appropriate radiographic diagnosis of implant fixation and/or fracture healing, but many cases remain challenging to diagnose.4

In the orthopaedic literature, there is no consensus on a reliable definition criterion for long-bone non-unions, making the standardization of diagnoses difficult; the lack of a trustworthy assessment for component fixation and fracture healing can lead to patients receiving sub-optimal care.5 This can also limit the collection of evidence supporting the use of specific implant components, surgical techniques, and post-operative activity guidelines. Moreover, comparing healing results of different clinical studies for long-bone non-unions can become problematic due to different criteria being used.5 Radiographic features associated with loosening such as radiolucencies are often only appreciable for the cement-bone interface, rather than the cement-implant interface which is the most common site of failure causing loosening.4 Failure to properly diagnose fracture non-union leaves patients in pain with substantial morbidity, while unnecessary surgery risks significant complication. Given the increasing rates of TKA and associated predictions of increasing revision burden,6 along with ongoing debates over the superiority of certain implants or surgical techniques, there is an unmet need for better fixation and bone healing assessment.

Historically, radiostereometric analysis (RSA) has been the gold standard for measuring implant fixation.7 However, it remains a niche tool limited to clinical research because of its requirement for implanted marker beads and specialized equipment being accessible for a handful of labs in North America.7-8 Recently, multiple groups have developed approaches to perform RSA-like measurements using clinical CT scans for shoulder, hip, and knee replacements.9 The accuracy and precision of the "CT-RSA" methods are on par with conventional RSA and acceptable for clinical studies.10-15 It is predicted that there will be a greater uptake of CT-RSA than conventional RSA, but the technology is still in its infancy.9 The application of CT-RSA will undoubtedly be more inclusive as examinations can now be performed on patients who did not have marker beads implanted at the time of their original surgery, and CAD models of implants are not needed.16

Using a weight-bearing CT scanner is the most similar implementation of CT-RSA to conventional RSA, as exams can be acquired in unloaded and loaded positions. However, the availability of weight- bearing CT will always be lesser than conventional clinical CT scanners, even as more and more high volume orthopaedic centres are acquiring weight-bearing CT scanners. While some groups have implemented specialized loading devices to perform such scans with conventional CT,17 a more generalizable approach of simple internal-external leg rotations held in place with tape would ultimately offer the greatest potential uptake across centres.

The results of this proposed study will demonstrate the ability to precisely measure displacements between bone segments under loading following periprosthetic fracture repair with weight-bearing CT and conventional CT scanners. These will be the first-ever measurements of distal femur periprosthetic fracture healing with weight-bearing CT-RSA. Demonstration of this will enable us to pursue future studies that are prospective in nature and may evaluate topics such as time to weight-bearing activities, different types of surgical reconstructions, and relationship between healing and bone quality/bone health. Other centres with access to weight-bearing CT will also benefit from this work. Consideration for the ability to perform similar measurements with conventional CT in place of weight-bearing CT will improve the generalizability of this approach and support it as a clinical diagnostic tool. For example, surgeons may better understand bone fragment motion over time and recommend appropriate postoperative activities for patients' weight-bearing tolerance. Therefore, the proposed study design will evaluate the ability to perform inducible displacement measurements following the surgical repair of distal femur periprosthetic fractures using both weight-bearing CT and conventional CT.

Inclusion criteria

• Underwent revision surgery for repair of a distal femur fracture surrounding a primary TKA femoral component using a fracture fixation plate >1 year
• Age 50-90 years
• Body mass index up to 40kg/m2
• Able to provide informed consent
• Able and willing to do study assessments and follow instructions

Exclusion criteria

• Does not understand English
• Undergone revision surgery for any other indication
• Received Intramedullary nail or distal femur replacement
• Cannot independently stand on one leg in the weight-bearing CT during the inducible displacement exam