Abstract
Biomechanical considerations are relevant to cup positioning in total hip replacement (THR) to optimise the patient-specific post-operative outcome. One goal is to place the hip centre of rotation (COR) such that parameters characterising the biomechanics of the hip joint lie within physiological ranges. Different biomechanical models have been developed and are based on exact knowledge about muscle insertion points whose positions can be estimated on the basis of bony landmarks. Therefore, accurate landmark localisation is necessary to obtain reliable and comparable parameter values.
As most biomechanical considerations are limited to the frontal plane, landmark localisation relying on standardised pre-operative radiographs has been established in clinical practice. One potential drawback of this approach is that user-interactive landmark localisation in radiographs might be more error-prone and subjective than localisation in 3D images. Therefore, we investigated the possibility of increasing the reproducibility of interactive landmark localisation by providing 3D localisation techniques. As the so-called BLB score based on Blumentritt's biomechanical hip model has already been introduced into clinical practice as a criterion for cup position planning, we examined the anatomical landmarks involved in BLB score evaluation. We developed a CT-based simulation tool allowing for the generation of 3D bone surface models and standardised digitally reconstructed radiographs (DRRs). Correspondences between points in the 2D DRR and rays in the 3D bone surface model are automatically established and optionally visualised by the tool.
Two modes of landmark localisation were examined: In the 2D-mode, only AP DRRs were displayed, and the users had to localise the landmarks by clicking within the DRR image. In the 3D-mode, additionally the arbitrarily rotatable bone surface models together with the aforementioned 2D/3D correspondences were visualised. The user could then choose between landmark localisation by clicking either within the DRR image or within the 3D view. In either case, the 2D landmark positions within the DRR were recorded.
The participants were given both an example AP pelvis radiograph with highlighted anatomical landmarks and the following landmark descriptions from the user's manual (v2.06) of the mediCAD software (Hectec GmbH, Landshut, Germany): P4: ca. 3cm distal lesser trochanter minor (in the imagined direction of pull of the rectus femoris muscle towards the medial upper edge of the patella); P5:lateral, most proximal edge of the trochanter major; P6: most cranial edge of the sclerotic area; P7:spina iliaca anterior inferior; P8/P9:most lateral/cranial point of the wing of the ilium.
(P1 and P2 are only needed to define the position of the mid-sagittal plane, and P3 is the pre-operative COR. Due to correct radiograph standardisation, we assumed this plane and P3 to be known prior to landmark localisation.)
Thirteen surgeons repeated the experiments on four hips (CT datasets of two male patients).
The following results were obtained (SD of relevant coordinates obtained with 2D localisation vs. SD of those obtained with 3D localisation) in the first patient (left hip: 1L; right hip: 1R) and the second patient (left hip: 2L; right hip: 2R):P4: 6.3 vs. 9.0 (1L); 6.7 vs. 5.6 (1R); 9.0 vs. 11.1 (2L); 7.1 vs. 8.6 (2R); P5: 4.4 vs. 2.8 (1L); 3.1 vs. 3.1 (1R); 4.3 vs. 2.4 (2L); 4.7 vs. 4.1 (2R); P6: 4.8 vs. 3.8 (1L); 2.9 vs. 2.8 (1R); 3.7 vs. 5.2 (2L); 6.9 vs. 3.5 (2R); P7: 12.2 vs. 6.1 (1L); 12.1 vs. 3.7 (1R); 7.6 vs. 4.6 (2L); 6.2 vs. 4.5 (2R); P8: 1.2 vs. 2.8 (1L); 2.0 vs. 2.6 (1R); 1.5 vs. 2.1 (2L); 2.0 vs. 1.6 (2R);P8: 4.1 vs. 2.1 (1L); 7.3 vs. 3.9 (1R); 1.6 vs. 2.6 (2L); 4.1 vs. 3.2 (2R).
The greatest differences in reproducibility were observed in P7, which was barely distinguishable in the radiographs and, hence, showed very low reproducibility only for the 2D-mode. P4 showed low reproducibility in both modes due to its vague description and the relatively small portions of the femurs contained in the CT-scanned volume. In P9 the low reproducibility obtained with the 2D-mode might be partly explained by truncation artefacts present in the DRRs.
Although our study needs to be extended to more datasets, we conclude that the availability of 3D data allows for higher landmark localisation reproducibility when compared with the conventional X-ray-based approach, which has additional drawbacks: Standardisation of X-ray imaging, which is necessary to retain comparability of biomechanical parameter values determined in different patients, is hard to achieve; specifications e.g. concerning the central beam may be met only after acquiring several radiographs. Moreover, once a 2D target cup position is defined based on the 2D biomechanical analyses, the transfer of this position into the 3D surgical site is difficult without additional 3D imaging.
Hence, the use of 3D imaging and 3D landmark localisation techniques seems more promising for cup positioning based on biomechanical models, which, however, need validation.
