Stefan Saevarsson

Radiological method for measuring patellofemoral tracking and tibiofemoral kinematics before and after total knee replacement

Objectives Numerous complications following total knee replacement (TKR) relate to the patellofemoral (PF) joint, including pain and patellar maltracking, yet the options for in vivo imaging of the PF joint are limited, especially after TKR. We propose a novel sequential biplane radiological method that permits accurate tracking of the PF and tibiofemoral (TF) joints throughout the range of movement under weightbearing, and test it in knees pre- and post-arthroplasty. Methods A total of three knees with end-stage osteoarthritis and three knees that had undergone TKR at more than one year’s follow-up were investigated. In each knee, sequential biplane radiological images were acquired from the sagittal direction (i.e. horizontal X-ray source and 10° below horizontal) for a sequence of eight flexion angles. Three-dimensional implant or bone models were matched to the biplane images to compute the six degrees of freedom of PF tracking and TF kinematics, and other clinical measures. Results The mean and standard deviation for the six degrees of freedom of PF tracking and TF kinematics were computed. TF and PF kinematics were highly accurate (< 0.9 mm, < 0.6°) and repeatable. Conclusions The developed method permitted measuring of in vivo PF tracking and TF kinematics before and after TKR throughout the range of movement. This method could be a useful tool for investigating differences between cohorts of patients (e.g., with and without pain) impacting clinical decision-making regarding surgical technique, revision surgery or implant design.

An experimental study of the interface pressure profile during level walking of a new suspension system for lower limb amputees

BACKGROUND Different suspension systems that are used within prosthetic devices may alter the distribution of pressure inside the prosthetic socket in lower limb amputees. This study aimed to compare the interface pressure of a new magnetic suspension system with the pin/lock and Seal-In suspension systems. METHODS Twelve unilateral transtibial amputees participated in the study. The subjects walked on a level walkway at a self-selected speed. The resultant peak pressure with the three different suspension systems was recorded using F-socket transducers. FINDINGS There were significant statistical differences between the three studied suspension systems. Pair-wise analyses revealed that the mean peak pressure (kPa) was lower with the magnetic system than it was with the pin/lock system over the anterior and posterior aspects during one gait cycle (89.89 vs. 79.26 and 47.22 vs. 26.01, respectively). Overall, the average peak pressure values were higher with the Seal-In system than they were with the new magnetic lock and pin/lock system. INTERPRETATION The new magnetic system might reduce the pressure within the prosthetic socket in comparison to the pin/lock and Seal-In system during one gait cycle. This is particularly important during the swing phase of gait and may reduce the pain and discomfort at the distal residual limb in comparison to the pin/lock system.

Are static and dynamic kinematics comparable after total knee arthroplasty

Knee kinematics provide information about how the femoral, tibial and patellar bones or prosthetic components move relative to each other. Accurate knowledge of kinematics is valuable for implant design, comparisons between designs or surgical techniques, and to identify differences between patients with good and poor outcomes. Both static and dynamic imaging techniques have been used to evaluate kinematics. In general, static imaging is used to capture better quality images or to capture views that cannot be acquired by dynamic imaging, whereas dynamic imaging is used to capture real-life movements. How well static kinematics represent dynamic kinematics is subject to frequent debate and has not been adequately addressed, especially after total knee arthroplasty (TKA). We compared the static and dynamic weightbearing kinematics of 10 female subjects after TKA. Using the same clinical scanner for both methods, static images were taken using our standard protocol, sequential-biplane radiographs at multiple flexion angles, as well as with dynamic video fluoroscopy during a step up activity. The static method can reliably measure all 12 degrees of freedom (DOF) after TKA, however only seven were compared due to the poorer out-of-plane reliability in the single-plane dynamic imaging. No differences were found between the static and dynamic kinematics for nine out of ten subjects. For one subject, however, a difference of 5-8° in internal/external tibial rotation was found. The research question, study purpose and the advantages and disadvantages of each method need to be considered when determining which imaging method to use.

Changes in knee kinematics following total knee arthroplasty

Total knee arthroplasty (TKA) changes the knee joint in both intentional and unintentional, known and unknown, ways. Patellofemoral and tibiofemoral kinematics play an important role in postoperative pain, function, satisfaction and revision, yet are largely unknown. Preoperative kinematics, postoperative kinematics or changes in kinematics may help identify causes of poor clinical outcome. Patellofemoral kinematics are challenging to record since the patella is obscured by the metal femoral component in X-ray and moves under the skin. The purpose of this study was to determine the kinematic degrees of freedom having significant changes and to evaluate the variability in individual changes to allow future study of patients with poor clinical outcomes. We prospectively studied the 6 degrees of freedom patellofemoral and tibiofemoral weightbearing kinematics, tibiofemoral contact points and helical axes of rotation of nine subjects before and at least 1 year after total knee arthroplasty using clinically available computed tomography and radiographic imaging systems. Normal kinematics for healthy individuals were identified from the literature. Significant differences existed between pre–TKA and post–TKA kinematics, with the post-TKA kinematics being closer to normal. While on average the pre–total knee arthroplasty knees in this group displayed no pivoting (only translation), individually only five knees displayed this behaviour (of these, two showed lateral pivoting, one showed medial pivoting and one showed central pivoting). There was considerable variability postoperatively as well (five central, two lateral and two medial pivoting). Both preop and postop, flexion behaviour was more hinge-like medially and more rolling laterally. Helical axes were more consistent postop for this group. An inclusive understanding of the pre–TKA and post–TKA kinematics and changes in kinematics due to total knee arthroplasty could improve implant design, patient diagnosis and surgical technique.

Kinematic Differences Between Gender Specific and Traditional Knee Implants

In the ongoing debate about gender-specific (GS) vs. traditional knee implants, there is limited information about patella-specific outcomes. GS femoral component features should provide better patellar tracking, but techniques have not existed previously to test this accurately. Using novel computed tomography and radiography imaging protocols, 15 GS knees were compared to 10 traditional knees, for the 6 degrees of freedom of the patellofemoral and tibiofemoral joints throughout the range of motion, plus other geometric measures and quality of life (QOL). Significant differences were found for patellar medial/lateral shift, where the patella was shifted more laterally for the GS femoral component. Neither group demonstrated patellar maltracking. There were no other significant differences in this well-functioning group.

Changes in Knee Shape and Geometry Resulting from Total Knee Arthroplasty

Changes in knee shape and geometry resulting from total knee arthroplasty can affect patients in numerous important ways: pain, function, stability, range of motion, and kinematics. Quantitative data concerning these changes have not been previously available, to our knowledge, yet are essential to understand individual experiences of total knee arthroplasty and thereby improve outcomes for all patients. The limiting factor has been the challenge of accurately measuring these changes. Our study objective was to develop a conceptual framework and analysis method to investigate changes in knee shape and geometry, and prospectively apply it to a sample total knee arthroplasty population. Using clinically available computed tomography and radiography imaging systems, the three-dimensional knee shape and geometry of nine patients (eight varus and one valgus) were compared before and after total knee arthroplasty. All patients had largely good outcomes after their total knee arthroplasty. Knee shape changed both visually and numerically. On average, the distal condyles were slightly higher medially and lower laterally (range: +4.5 mm to −4.4 mm), the posterior condyles extended farther out medially but not laterally (range: +1.8 to −6.4 mm), patellofemoral distance increased throughout flexion by 1.8–3.5 mm, and patellar thickness alone increased by 2.9 mm (range: 0.7–5.2 mm). External femoral rotation differed preop and postop. Joint line distance, taking cartilage into account, changed by +0.7 to −1.5 mm on average throughout flexion. Important differences in shape and geometry were seen between pre-total knee arthroplasty and post-total knee arthroplasty knees. While this is qualitatively known, this is the first study to report it quantitatively, an important precursor to identifying the reasons for the poor outcome of some patients. Using the developed protocol and visualization techniques to compare patients with good versus poor clinical outcomes could lead to changes in implant design, implant selection, component positioning, and surgical technique. Recommendations based on this sample population are provided. Intraoperative and postoperative feedback could ultimately improve patient satisfaction.