THA is one of the most conventional orthopaedic reconstructive procedures. Significant improvement in functional outcomes and prosthesis survival have been achieved due to the advances in surgical techniques and bioengineering technology [8–11]. However, the long-term survivial of prostheses and the occurrence of postoperative complications remain challenges in THA . Research indicates that the survival rate of the femoral component is higher than that of the acetabular component, and the prosthesis lifespan mainly relies on the survival of the acetabular component in THA. Therefore, the orientation of the acetabular cup has a significant effect on the long-term outcome of THA.
Traditionally, depending on a surgeon‘s subjective decision and experience ， the acetabular cup is inserted within the range of the “safety zone” intraoperatively [14–18],it is acceptable that an acetabular prosthesis is inserted with an abduction angle ranging from 40° to 60° and an anteversion angle ranging from 5° to 25°. However, this approach not always results in accurate placement. The computer navigation system optimizes the implantation of prosthesis, but it can be easily influenced by the pelvic position , which results in a combination of errors in registration, landmark identification, and device tracking .The cost of navigation system must also be taken into consideration.
Generally, the orientation of the cup is determined based on its anteversion and abduction. In 3D space, the acetabular central axis indicates the opening orientation of the acetabulum. The central axis of the acetabular prosthesis should coincide with the anatomical acetabular central axis. Thus, the physiological and biomechanical environment of the hip joint can be restored optimally. Therefore, determining the acetabular centre of rotation and the central axis is the basis of biomechanical restoration in THA.
In general, it is difficult to locate the central axis during the operation because of variations in the center of the acetabulum due to osteoarthritis. However, it was easy for the “mirror image” of the normal side to serve as a reference in this study. In this case, we obtained the rotational centre of the contralateral side, which could then be considered as the centre of the diseased side. Using the anatomical landmarks for the measurements which can be clearly marked on the projections of the pelvis, the anteversion and abduction angles can be obtained, and errors can be minimized.
In this study, the coordinates of the acetabular central axis were determined by two points, one point being the centre of the contralateral acetabulum, and the other point being point “L”,which was calculated using trigonometric function named after the senior author LJL. Point “L” is a unique point related to the radius of the contralateral acetabulum, it is located on the acetabular central axis and can be seen as a site spatially transferred from the centre of the acetabulum. By constructing an appropriate reference frame, we can easily calculate the spatial straight-line equation of the acetabular central axis and the coordinates of point”L” .
We chose the acetabular fossa as the reference for the navigation template because it has many advantages, such as little variation in shape, no articular cartilage covering the surface, and stable and unique bony features. Therefore, an accurate fit can be achieved. We could also simulate the postoperative view using software and assess the location of the prosthesis and the validity of the navigational template.
Therefore, our patient-specific navigational template is easy to apply intraoperatively without much special training or a steep learning curve,in contrast with complex computer navigation systems. This template can meet the needs of different individuals and decreases the duration of operation. The time required for design and manufacturing,as well as the cost,are acceptable for most patients. Therefore, our template is a simple and low-cost solution that can be precisely, safely, and rapidly implementated in unilateral THA.
Certainly, there are several limitations to this research. First, in the case of deformity on the contralateral side, e.g. that in pathological conditions such as dysplasia and post-traumatic malunion,among others, the method we propose may not be very effective. Severe dysplasia and osteoarthritis on the operation side, which may lead to a deformed acetabular fossa, can also cause inaccurate placement of the acetabular cup. Second, this was a cadaveric study, and the proposed method has not been applied in clinical practice. Furthermore, 20 is a small number compared with the number of THA surgeries every year, thus, the results may not represent the various abnormalities in such a population. Third, errors in the designing and RP procedures are inevitable, and the symmetry of bilateral acetabular fossae of bilateral sides needs to be further researched.