ArthroPlanner: A Surgical Planning Solution for Acromioplasty

Introduction Subacromial impingement of the rotator cuff between the anterior [1] or lateral acromion [2] and the superior humeral head is a common disorder. This condition arises when the subacromial space height is too narrow during active elevation or scaption of the arm above shoulder level due to an abnormal hooked shape or large lateral extension of the acromion.

In severe cases of impingement syndrome, an arthroscopic acromioplasty surgery is usually performed to resect the different area of the acromion causing damage to the subacromial structures. The exact location and the amount of bone to be resected is generally left to the unique appreciation of the orthopedic surgeon during surgery. To improve the precision of this resection, surgeons could greatly benefit from a surgical planning solution that aims at providing precise information about the surgical procedure. Moreover, since subacromial impingements are the result of a dynamic mechanism, an effective planning solution should analyze both the morphological joint's structures and its dynamic behavior during shoulder movements to fully apprehend the patient joint's condition.

Computer-assisted planning solution "ArthroPlanner" for acromioplasty is nowadays available. The solution allows to perform standard morphological bony measurements, as well as 3D simulations of the patient's joint during everyday shoulder activities. The software computes the precise bone resection (location and amount) based on detected subacromial impingements during motion.

The goal of this study was thus to compare clinical and radiological results of superior rotator cuff repair with or without computer-assisted planning. The hypothesis was that preoperative planning of acromioplasty would allowed more accurate bone resection, would decrease postoperative impingements and consequently improved postoperative range of motion and tendon healing.

Methods

We reconstruct the bones of the patient's shoulder joint (scapula and humerus from the humeral head to the mid-shaft) from a CT image using Mimics software (Materialise NV, Leuven, Belgium). The bones are then imported into ArthroPlanner software and the following steps are performed:

First, generic bone models are produced using a template fitting approach that deforms a bone template with an optimized topology (one for the scapula and one for humerus) to the reconstructed bone. This allows us in the next steps to exploit anatomical correspondences and to automatize landmarks and points selection on the mesh.

Second, biomechanical parameters are computed to permit motion description of the glenohumeral joint. The glenohumeral joint center is automatically calculated by a sphere fitting technique [3] that fits a sphere to the humeral head using the points of the proximal humerus model. Bone coordinate systems are established for the scapula and humerus. based on the definitions suggested by the International Society of Biomechanics [4] using anatomical landmarks defined on the bone models. Missing landmarks such as the lateral and medial epicondyles are identified on the CT image.

Third, morphological measurements are performed to analyze individual shoulder anatomy. The Critical Shoulder Angle [5] and the β angle [6] are calculated, as they are criteria associated with rotator cuff tears. The angles are computed in 3D based on bony landmarks and can be, if necessary, interactively adjusted by the user by manipulating 3D handles in the viewer.

Fourth, motion is applied at each time step to the humerus model with real-time evaluation of impingement. The minimum humero-acromial distance that is typically used for the evaluation of subacromial impingement is measured [7]. This distance is calculated in millimeters based on the simulated bones models positions. A color scale is also used to map the variations of distance on the scapula surface (red color = minimum distance, other colors = areas of increased distance). Given the thickness of the potential impinged tissues, subacromial impingement is considered when the computed humero-acromial distance is \< 6 mm, as suggested in the literature [7]. To test a wide variability of realistic movements, a motion database of daily activities (e.g., cross arm, comb hair) is used in addition to standard kinematic sequences (e.g., elevation, scaption).

Finally, the acromial resection plan is defined based on the 3D simulation results. A color map is used to represent areas where impingements occurred between the acromion and humerus (Fig. 1D). The red color denotes the area with the smallest humero-acromial distance computed over the different motion simulations.

The results at each step of the planning procedure are carefully validated by the user before continuing to the next ones. At the end of the planning, a PDF report is generated that contains patient's information and the measurements performed. The bones and the simulation data are also exported to be used in a simple 3D viewer (Fig. 2) dedicated to the surgeon. With this viewer, the surgeon is able to play all simulations, observe impingements dynamically and review the resection plan.