Accurate spatial location of joint center (JC) is a key issue in motion analysis since JC locations are used to define standardized anatomical frames, in which results are represented. Accurate and reproducible JC location is important for data comparison and data exchange. This paper presents a method for JC locations based on the multiple regression algorithms without preliminary assumption on the behavior of the joint-of-interest. Regression equations were obtained from manually palpable ALs on each bone-of-interest. Results are presented for all joint surfaces found on the clavicle, scapula and humeral bone. Mean accuracy errors on the JC locations obtained on dry bones were 5.2±2.5 mm for the humeral head, 2.5±1.1 mm for the humeral trochlea, 2.3±0.9 mm for the humeral capitulum, 8.2±3.9 mm for the scapula glenoid cavity, 7.2±3.2 mm for the scapular aspect of the acromio-clavicular joint, 3.5±1.8 mm for the clavicular aspect of the sternoclavicular joint and 3.2±1.4 mm for the clavicular aspect of the acromio-clavicular joint. In-vitro and in-vivo validation accuracy was 5.3 and 8.5 mm, respectively, for the humeral head center location. Regression coefficients for joint radius dimension and joint surface orientation were also processed and reported in this paper.
Spatial location of anatomical landmarks (ALs) is often required to quantify various aspects of skeletal morphology. ALs are required to define anatomical frames to represent movement data according to conventions (Cappozzo et al., 2005). Most ALs can be located either by manual palpation or virtual palpation on three-dimensional (3D) models, or a combination of both (Van Sint Jan et al., 2006; Van Sint Jan, 2007). Key ALs like joint center (JC) positions are required to meet standards (Wu and Cavanagh, 1995; Wu et al., 2002 and Wu et al., 2005) and are not palpable. Despite their usefulness, previously published methods use to estimate JCs show shortages. Functional methods are recognized to be the most accurate method (Stokdijk et al., 2000). They use particular movements to determine mechanical JCs and seem satisfactory for ball-and-socket joints in normal conditions (Camomilla et al., 2006; Cappozzo, 1984; Leardini et al., 1999). Unfortunately, they require performing motion patterns that are often too complex for patients showing limited joint amplitude (Di Silvestro et al., 2007). Morphological methods use spatial locations of palpable ALs to determine JCs using regression analysis (Barbaix et al., 2000; Bell et al., 1990; Meskers et al., 1998). Morphological methods seem therefore more appropriate because the analyzed individual patient can remain at rest during the palpation. To allow reproducible JC construction, ALs must lie close to the skin surface to be relatively accessible to palpation and must be readily identifiable even by less experienced investigators. Past effort developed strict AL definition to improved AL palpation reproducibility (Van Sint Jan, 2007). Recent findings reported that manual palpation of the humerus and scapula ALs can be achieved with a satisfactory accuracy (Van Sint Jan et al., 2007). For the shoulder complex, some morphological methods assume that the scapula glenoid cavity and the humeral head share the same center (Barbaix et al., 2000; Bell et al., 1989 and Bell et al., 1990; Meskers et al., 1998). In such method, the shoulder JC is determined from scapula ALs. This seems acceptable for shoulder joints showing normal behavior, but less acceptable for pathological instable joints (Grant et al., 2007). In such conditions, one cannot assume that the relationships between scapular ALs and the humeral head center remain constant during movements occurring in the glenohumeral joint (Yamamoto et al., 2007). Some authors reported that the glenohumeral joint does not behave like a pure ball-and-socket joint in normal conditions, and that physiological laxity is observable (Hatzel et al., 2006). This contradicts the above equivalence assumption. Joint size estimation is also of interest for arthroplasty (Harryman et al., 1995).
This study aimed to determine if the position of the morphological shoulder JCs can be interpolated with satisfactory accuracy from enough ALs palpated on the bone-of-interest itself. This statement was made possible after the availability of detailed palpation definitions of skeletal ALs (Van Sint Jan, 2007). Such standardized AL definitions allowed increasing palpation repeatability (Van Sint Jan and Della Croce, 2005). Results, including in-vitro and in-vivo validation, for the shoulder joint complex bones are presented. The main goal of this paper was obtaining regression relationships applicable for in-vivo shoulder motion analysis. This method would allow estimating patient's joint surface center, orientation and dimensions without a-priori knowledge on the functional aspect of the joint. The method was then compared with other morphological methods applied on the shoulder (Meskers et al., 1998).
