The posterior midline approach used in spinal surgery has been associated with a significant rate of wound dehiscence. This study investigates anatomical study of the arterial supply of the cervical and thoracic spinal muscles and overlying skin at each vertebral level. It aimed to provide possible anatomical basis for such wound complications.

A dissection and angiographic study was undertaken on 8 cadaveric neck and posterior torso from 6 embalmed and 2 fresh human cadavers. Harvested cadavers were warmed and hydrogen peroxide was injected into the major arteries. Lead oxide contrast mixture was injected in stepwise manner into the subclavian and posterior intercostal arteries of each specimen. Specimens were subsequently cross-sectioned at each vertebral level and bones elevated from the soft tissue. Radiographs were taken at each stage of this process and analysed.

The cervical paraspinal muscles were supplied by the deep cervical arteries, transverse cervical arteries and vertebral arteries. The thoracic paraspinal muscles were supplied by the superior intercostal arteries, transverse cervical arteries and posterior intercostal arteries. In the thoracic region, two small vessels provide the longitudinal connection between the segmental arteries and in the cervical region, deep cervical arteries provide such connection from C3 to C6. The arterial vessels supplying the paraspinal muscles on the left and right side anastomose with each other, posterior to the spinous processes in all vertebral levels. At cervical vertebral levels, source arteries travel near the surgical field and are not routinely cauterised; Haematoma is postulated to be the cause of wound complications. At thoracic levels, source arteries travel in the surgical field and tissue ischemia is a contributing factor to wound complications, especially in operations over extensive levels.

Post-operative wound complications is a multi-factorial clinical problem, the anatomical findings in this study provide possible explanations for wound dehiscence in the posterior midline approach. It is postulated that drain tubes may reduce the incidence of haematoma in the cervical level.

Shoulder girdles of 20 cadavers (68–94yrs) were harvested. The anterior (ACHA) and posterior circumflex humeral arteries (PCHA) were injected with ink and the extra and intraosseous courses of the dyed vasculature dissected through the soft tissues and bone to the osteotendinous junctions of the rotator cuff. The ink injection and bone dissection method was newly developed for the study.

Rates of cross-over at the osteotendinous juntion were 75% in the supraspinatus, 67% in subscapularis, 33% in infraspinatus and 20% in teres minor. The supraspinatus and subscapularis insertions were vascularised by the arcuate artery, a branch of the ACHA. The insertions of the infraspinatus and teres minor were supplied by an unnamed terminal branch of the PCHA.

The insertions of the rotator cuff receive an arterial supply across their OTJ's in 50% of cases. This may explain observed rates of AVN in comminuted proximal humeral fractures. The terminal branch of the PCHA supplying the infraspinatus and teres minor insertions was named the “Posterolateral Artery”. Finally, the new method employed for this study which allowed for direct visualisation of intraosseous vasculature, will enhance our understanding of skeletal vascular anatomy and have clinical applications in orthopaedic and reconstructive surgery.

Mechanical tests that have been carried out to validate finite-element models predicting vertebral strength concern vertebral bodies under axial compression. But in standing position gravity loads can induce a flexion component, especially for the last thoracic and first lumbar vertebrae. The aim of the study was to evaluate the strength of complete vertebrae under anterior compression.

15 isolated vertebrae T11-L2 (four women, one man, 88 ± 14 years old) were tested to failure. The load was applied at the one third of the vertebral body depth through a ball constrained in a hole. It was homogeneously distributed on the vertebral endplate through a polymetylmetacrylate (PMMA) layer which completely fills the concavity. The solid composed by the PMMA layer and the steel plate containing the hole for the ball was called “upper plate”. Its 3D orientation was assessed using the Polaris® motion capture system (accuracy: 0.6 mm, 0.6°) thanks to tripods. Before testing, the position of the marker-frames was assessed using 3D reconstructions (obtained by bi-planar X-rays) to express all the movements relatively to the vertebral frame.

The outcome data was the position of the upper plate. The load was calculated from the measurement of the vertical load (using the testing machine sensor) and the orientation of the upper plate (using the Polaris® system).

The mean flexion of the upper-plate is equal to 1° (± 0.7°) before the vertebra collapses. As this value is weak, the optoelectronic assessment could be removed during the test if the initial 3D orientation of the upper plate relatively to the vertebral frame is assessed.

This protocol allowed collecting with accuracy all the data necessary to validate models.

One of the advantages of Computer Assisted Orthopaedic Surgery is to obtain functional and morphological information in real time during the procedure. 3D models can be built, without preoperative images, based on elastic 3D to 3D registration methods. The bone morphing algorithm is one of them. It allows to specifically build the 3D shape of bones using a deformable model and a set of spare points obtained on the patient. These points are obtained with a pointer tracker visible by the station which digitises the surface of the bone. However, it’s not always possible to digitise directly the bone in the context of minimal invasive surgery. In this case, the lack of information leads to an inaccurate reconstruction of bone’s surfaces. To collect such missing information we propose to rely on ultrasound (US) images despite the fact that ultrasound is not the best modality to image bones.

To use this method, a segmentation step is first needed to detect automatically the bone in US images. Then, a calibration step of the US probe is carried out to obtain the 3D position of any point of the 2D ultrasonic images using 3D infra-red localizer. Several methods can be carried out to calibrate US probes, however to take into account surgical constraints such as accuracy, robustness, speed and ease of use, we decided to implement the single wall procedure.

The calibration step consists in the estimation of a transformation matrix which carries out the connection between the 2D reference system of the US image and a 3D reference system in the space. To estimate correctly this matrix, a wall is scanned with different motions of the US probe. The images are then processed to automatically detect the lines representing the wall in the US images. A preliminary step allows to clean the images using a threshold and a gradient operation. Then, a method based on the Hough transform detects the lines on the images. Once all the images are processed, the calibration parameters can be estimated by using a new method which minimises the distance between the real plane and the points obtained with the US images. This optimisation step is composed of the genetic algorithms and of the Levenberg-Marquardt (LM) method. The first algorithm allows to obtain a good initialisation in a defined space for the LM algorithm. This good initialisation found thanks to the stochastic behaviour of the genetic algorithms is very important otherwise the LM algorithm could detect local minimum and the calibration parameters could be wrong.

The accuracy of the calibration method is assessed by measuring the distance between the position of a known point in the space and the same point obtained with the US image and the calibration. 40 calibrations matrices are used to estimate correctly the accuracy. An average accuracy of 1.22 mm and a standard deviation (Std. Dev.) of 0.42 mm are measured. The accuracy of the system is quite high but the reproducibility is too low to use this approach in a clinical environment. The main reason of this lack of reproducibility is the thickness of the US beam.

A slight modification in the design of the calibration tool will allow to increase the reproducibility. We will then have an efficient and automatic calibration procedure with the required accuracy and robustness, usable for clinical purposes.