Background

The distal part of the radius is the most common localisation of fractures of the human body. Dislocated intraarticular fractures of the distal radius (FDR) are frequently treated by open reduction and internal fixation with a volar locking plate (VLP) under fluoroscopic guidance. Typically the locking screws are placed subchondral near the joint line to achieve maximum stability of the osteosynthesis. To avoid intraarticular screw placement an intraoperative virtual implant planning system (VIPS) as an application for mobile C-arms was established. The aim of the study was the validation of the implemented VIPS comparing the intraoperative planning with the actual placement of the screws. The study was conducted as a single-centre randomised controlled trial in a primary care institution. The hypothesis of the study was that there is conformity between the virtual implant position and the real implant placement.

Background

Digital planning of implants in regard to position and size is done preoperatively in most cases. Intraoperative it can only be made by navigation systems. With the development of the VIPS-method (Virtual Implant Planning System) as an application for mobile C-arms, it is possible to do an intraoperative virtual planning of the screws near the joint in treatment of distal radius fractures by plating. Screw misplacement is a well known complication in the operative treatment of these fractures. The aim of this prospective randomised trial was to gain first clinical experiences and to compare VIPS with the conventional technique. The study hypothesis was that there will be less screw misplacement in the VIPS group.

The goal of this study was to validate accuracy and reproducibility of a new 2D/3D reconstruction-based program called “HipRecon” for determining cup orientation after THA. “HipRecon” uses a statistical shape model based 2D/3D deformable registration technique that can reconstruct a patient-specific 3D model from a single standard AP pelvic X-ray radiograph. Required inputs include a digital radiograph, the pixel size, and the film-to-source distance. No specific calibration of the X-ray, or a CAD (computer-assisted design) model of the implant, or a CT-scan of the patient is required. Cup orientation is then calculated with respect to the anterior pelvic plane that is derived from the reconstructed 3D-model.

The validation study was conducted on datasets of 29 patients (31 hips). Among them, there were 15 males and 14 females. Each dataset has one post-operative X-ray radiograph and one post-operative CT-scan. The post-operative CT scan for each patient was used to establish the ground truth for the cup orientation. Radiographs with deep centering (7 radiographs), or of pelvises with fractures (2 radiographs), or with both (1 radiograph), or of non-hemispherely shaped cup (1 radiograph) were assessed separately from the radiographs without above mentioned phenomena (18 radiographs) to estimate a potential influence on the 2D/3D reconstruction accuracy. To make the description easier, we denote those radiographs with above mentioned phenomena as non-normal cases and those without as normal cases. The cup anteversions and inclinations that were calculated by “HipRecon” were compared to the associated ground truth. To validate the reproducibility and the reliability, one observer conducted twice measurements for each dataset using “HipRecon”.

The mean accuracy for the normal cases was 0.4° ± 1.8° (−2.6° to 3.3°) for inclination and 0.6° ± 1.5° (−2.0° to 3.9°) for anteversion, and the mean accuracy for the non-normal cases was 2.3° ± 2.4° (−2.1° to 6.3°) for inclination and 0.1° ± 2.8° (−4.6° to 5.1°) for anteversion. Comparing the measurement from the normal radiographs to those from the non-normal radiographs using the Mann-Whitney U-test, we found a significant difference in measuring cup inclination (p = 0.01) but not in measuring cup anteversion (p = 0.3). Bland-Altman analysis of those measurements from the normal cases indicated that no systematical error was detected for “HipRecon,” as the mean of the measurement pairs were spread evenly and randomly for both inclination and anteversion. “HipRecon” showed a very good reproducibility for both parameters with an intraclass correlation coefficient (ICC) for inclination of 0.98 (95% Confidence Limits (CL): 0.96–0.99) and for anteversion of 0.96 (95% CL: 0.91–0.98).

Accurate assessment of the acetabular cup orientation is important for evaluation of outcome after THA, but the inability to measure acetabular cup orientation accurately limits one's ability to determine optimal cup orientations, to assess new treatment methods of improving acetabular cup orientation in surgery, and to correlate the acetabular cup orientation to osteolysis, wear, and instability. In this study, we showed that “HipRecon” was an accurate, consistent, and reproducible technique to measure cup orientation from post-operative X-ray radiographs. Furthermore, our experimental results indicated that the best results were achieved with the radiographs of non-fractured pelvises that included the anterior superior iliac spines and the cranial part of the non-fractured pelvis. Thus, it is recommended that these landmarks should be included in the radiograph whenever the 2D/3D reconstruction-based method will be used

Navigation is the combination of real and virtual anatomy. Registration brings the virtual world of imaged anatomy into accordance with the real world of actual anatomy. Without a navigation system the process takes place in the head of the surgeon, he assigns the image data to the patient’s anatomy, based on his experience. This process is called mental registration.

Registration methods: Every point in the patient’s anatomy correlates to one point in the three-dimensional image of the patient. Every point in the anatomy and the image can be clearly defined as a position vector in a Cartesian coordinate system. Registration is carried out by a series of transformations of the different Cartesian coordinate systems. The registration between the real and the virtual world can be performed manually or automatically. Different technologies are available for this process.

In the Paired Points Matching landmarks, which can be clearly identified in the image dataset and in the in situ anatomy, are registered pre-operatively in a three-dimensional (e.g. CT) dataset. At least three points, registered as precisely as possible in the dataset and the intra-operative anatomy, are necessary to define the spatial position of the dataset and to bring it into correlation with the patients anatomy. On the spine, the existing prominent landmarks on the accessible dorsal part of the vertebrae, the dorsal process, and the joint condyles are used. Different factors contribute to an inaccurate registration, like an inadequate preparation of the anatomic structure, a misinterpretation of the landmarks by the surgeon, or anatomic variations, that formed in the time between the CT images and the operation.

The definition of corresponding point pairs can be difficult in many applications and an increased degree of invasivity must be accepted. Therefore, the precise recognition of the predefined points of the image dataset in the patient’s anatomy is severely impaired. However, other characteristics, e.g. curves or surfaces of bones can be extracted from the image data. These form the basis for the Surface Matching. A series of points on the surface of the bone must then be digitalised intra-operatively. This accumulation of points is then transferred to the corresponding virtual surface with the help of a complex mathematic algorithm, so that the gap between the points and the surfaces is minimized.

Under special circumstances the registration can be carried out automatically. For this it is necessary that the position of both coordinate systems is known at the time of image recording. To do this, a reference array needs to be attached to the patient and thus the automatic registration can only be performed intra-operatively. In general, all available intra-operative imaging equipment can be used.

Bulky equipment, such as computer tomography or magnetic resonance imaging is available intra-operatively only in very few facilities. The most valuable source for intra-operative images is the image intensifier. Images can be recorded with a navigated, calibrated C-Arm in the standard positions relevant for the surgery. Several fluoroscopic image layers can be displayed at the same time as optical information in the operating room in the form of a permanent virtual fluoroscopy.

Since 2001 a fluoroscopic image intensifier is available, which can generate three-dimensional multilayer reconstructions of high-contrast objects, like bones, from single fluoroscopic images. Since the introduction of three-dimensional imaging techniques in navigation, it is possible to perform the automatic registration of three-dimensional data. So, the above described limitations of CT-based navigation for minimal invasive surgery, e.g. not being able to update the dataset and errors during manual registration, were taken into account. However, the process of automatic registration is highly complex and influenced by many factors.

Surgical treatment of pelvic injuries is one of the most challenging tasks in trauma surgery. Intra-operative two-dimensional imaging technology can often not cope with the complex requirements of the three-dimensional anatomy of the pelvis. A registration, which is difficult to achieve with minimal invasive techniques, is obligatory for the CT-based navigation. Changes in the reduction can only be visualized inadequately. The intra-operative imaging after completed osteosynthesis has significantly enhanced since the introduction of three-dimensional image amplifiers. The three-dimensional data can be used directly for the visualization of the osteosynthesis material by linking it to a navigation system.

Since January 2001 the Trauma Center Ludwig-shafen has the ability to perform the registration-free three-dimensional navigation by linking the 3D image intensifier to a navigation system. From January 2002 to January 2005 30 patients with a pelvic injury, where the intra-operative navigation was carried out with the 3D image intensifier, were included in a prospective study. A complete neurological status, conventional fluoroscopic diagnosis, and CT-images were available pre-operatively for all patients. This information formed the basis for the classification and indication for surgery. Patients were positioned on a metal-free carbon table. Due to the registration-free navigation, and thus without the need for a manual registration of landmarks, a tissue-saving preparation could be performed. The postoperative assessment of the implant position was carried out by an independent radiologist.

Screw placement on the pelvic ring was performed in 23 patients (IS lag screws), in 3 patients on both sides. Periacetabular screws were implanted in 7 patients with acetabular fractures. A prerequisite was that the closed repositioning and a temporary fixation could be carried out before the recording of the 3D dataset. 7 surgeons participated in this study. The 3D image intensifier and the navigation system were always operated by the same person. In total 66 screws were implanted (49 IS screws, 17 periacetabular screws). One misplacement of a IS screw with a penetration of the neuroforamen was found during post-operative check-ups. The screw position was corrected during revision surgery. The mean fluoroscopy time for the recording of the 3D scans and the 2D check-ups was 1.78 (+/− 0.4) min. The mean operating time was 105 (+/− 24) min.

This prospective study demonstrated the clinical use of navigation in a three-dimensional dataset from the 3D image intensifier with automatic registration on the pelvis. A relatively high misplacement ratio during IS lag screw placement in the traditional, percutaneous technique according to Matta up to 30% is described in literature. The 3D image intensifier navigation facilitates a standardized working process in the operating room. This is reflected in the low range in fluoroscopy and operating time. The limiting factor in pelvic surgery is the relatively small image volume of the 3D image intensifier of 12 cm3 and the low image quality compared to a CT.

After experimental and preclinical evaluation (HAP Paul Award 2001) of a CT-free image guided surgical navigation system for acetabular cup placement, the system was introduced into clinical routine. The computation of the angular orientation of the cup is based on reference coordinates from the anterior pelvic plane concept. A hybrid strategy for pelvic landmark acquisition has been introduced involving percutaneous pointer-based digitisation with the non-invasive bi-planar landmark reconstruction using multiple registered fluoroscopy images.

From January 2001 to May 2002 a total of 118 consecutive patients (mean age 68 years, 82 male, 36 female, 62 left and 56 right hip joints) were operated on with the hybrid CT-free navigation system. During each operation the angular orientation of the inserted implant was recorded.

To determine the placement accuracy of the acetabular components the first 50 consecutive patients underwent a CT scan seven to ten days postoperatively to analyse the cup position related to the anterior pelvic plane. This was done blinded with commercial planning software. There was no significant learning curve observed for the use of the system.

Mean values for postoperative inclination read 43° (SD 3.0, range 37 to 49) and anteversion 19° (SD 3.9, range 10 to 28). The resulting system accuracy, i.e., the difference between intraoperatively calculated cup orientation and postoperatively measured implant position shows a maximum error of 5° for the inclination (mean 1.5°, SD 1.1) and 6° for the anteversion (mean 2.4°, SD 1.3).

An accuracy of better than 5° inclination and 6° ante-version was achieved under clinical conditions, which implies that there is no significant difference in performance from the established CT-based navigation methods. Image guided CT-free cup navigation provides a reliable solution for future THA.