No abstract

Measuring the minimum-joint-space-width (mJSW) in total knee arthroplasty (TKA) in Roentgen stereophotogrammetric analysis (RSA) provides valuable information on polyethylene wear, a leading cause for TKA failure. Most existing studies use nonweight-bearing (NWB) patient positioning. The latter may compromise mJSW measurements due to knee laxity with subsequent noncontact between the TKA components. We investigated the difference in mJSW between weight-bearing (WB) and NWB images and the association with mediolateral (ML) knee stability. At one-year follow-up, 23 TKAs were included from an ongoing RSA study, and ML stability was evaluated. For each examination, the mJSW and femoral-tibial contact locations were measured. A linear regression model was used to analyze the association between the mJSW difference (NWB-WB) with the ML stability and contact locations. The mean mJSW difference was 0.28 mm medially and 0.20 mm laterally. Four TKAs had medium (5-9˚) and 19 TKAs had high (<5˚) ML stability. A higher mJSW difference was found for TKAs with medium stability (0.36 mm, P ¼ 0.01). In conclusion, mJSW measurements in existing (NWB) RSA studies are influenced by knee laxity, but may still provide information on wear progression based on TKA with high ML stability. A direct comparison of mJSW measurements from WB and NWB data is not possible. ß

No abstract

ObjectivesAn important measure for the diagnosis and monitoring of knee osteoarthritis is the minimum joint space width (mJSW). This requires accurate alignment of the x-ray beam with the tibial plateau, which may not be accomplished in practice. We investigate the feasibility of a new mJSW measurement method from stereo radiographs using 3D statistical shape models (SSM) and evaluate its sensitivity to changes in the mJSW and its robustness to variations in patient positioning and bone geometry.Materials and MethodsA validation study was performed using five cadaver specimens. The actual mJSW was varied and images were acquired with variation in the cadaver positioning. For comparison purposes, the mJSW was also assessed from plain radiographs. To study the influence of SSM model accuracy, the 3D mJSW measurement was repeated with models from the actual bones, obtained from CT scans.ResultsThe SSM-based measurement method was more robust (consistent output for a wide range of input data/consistent output under varying measurement circumstances) than the conventional 2D method, showing that the 3D reconstruction indeed reduces the influence of patient positioning. However, the SSM-based method showed comparable sensitivity to changes in the mJSW with respect to the conventional method. The CT-based measurement was more accurate than the SSM-based measurement (smallest detectable differences 0.55 mm versus 0. 82 mm, respectively).ConclusionThe proposed measurement method is not a substitute for the conventional 2D measurement due to limitations in the SSM model accuracy. However, further improvement of the model accuracy and optimisation technique can be obtained. Combined with the promising options for applications using quantitative information on bone morphology, SSM based 3D reconstructions of natural knees are attractive for further development.Cite this article: E. A. van IJsseldijk, E. R. Valstar, B. C. Stoel, R. G. H. H. Nelissen, N. Baka, R. van’t Klooster, B. L. Kaptein. Three dimensional measurement of minimum joint space width in the knee from stereo radiographs using statistical shape models. Bone Joint Res 2016;320–327. DOI: 10.1302/2046-3758.58.2000626.

Implant failure related to polyethylene wear remains an important issue in total knee arthroplasty. Polyethylene wear is usually assessed in vivo by measuring the remaining insert thickness on X-ray images of the knee. To reflect the amount of wear debris more accurately, a 3-dimensional overlap measurement has been suggested, which is based on implant component models which are matched on calibrated stereo X-ray images using model-based roentgen stereophotogrammatic analysis. The goal of this study was to determine the influence of pose estimation, insert thickness deviation and variation in the femoral-tibial contact location on the accuracy and precision of the measurement using simulations and a phantom experiment. We found that the pose estimation was the largest source of variation. The 95% prediction interval varied between 111 and 283 mm(3), which is approximately 100-200% of the detected volumetric wear. Insert thickness variation resulted in prediction intervals of 74-174 mm(3). Variation of the femoral-tibial contact location in the phantom experiment gave a prediction interval of 40 mm(3). Large differences in the detected wear volume were found for different flexion angles. At most 56% of the true wear volume was detected (129 of 230 mm(3), 30° of flexion). In summary, both the accuracy and precision of the volumetric wear measurement were low. The prediction interval of the volumetric wear measurement is at least as large as the measurement outcome itself. This is an important limitation to the applicability of the volumetric wear measurement in clinical practice and further clinical validation is required.