Distribution Of Subchondral Mineralization Research Articles

The Influence of Posterior Instrumentation on Adjacent and Transfixed Facet Joints in Patients With Thoracolumbar Spinal Injuries: A Morphological in Vivo Study Using Computerized Tomography Osteoabsorptiometry

Subchondral mineralization of adjacent and transfixed facet joints was analyzed in patients with thoracolumbar spinal injuries, both before posterior instrumentation and after removal of the spinal implant. To examine the influence of posterior instrumentation on content and distribution of subchondral mineralization as a correlate of the long-term load acting on the adjacent and transfixed zygapophysial joints. Posterior stabilization plays an important role in the treatment of spinal injuries and is a standard technique for the treatment of thoracolumbar spinal fractures. Studies have shown that stress and motion in the adjacent segments are altered in the presence of instrumentation. Twenty-three patients with thoracolumbar spinal injuries had computerized tomography (CT) during the course of routine posttraumatic diagnostics and subsequently received bisegmental posterior fixation with an internal fixator. Second CT were obtained after removal of the fixation device, which was performed on an average of 9.4 months after the trauma. Patients were divided into 2 groups with follow-up CT taken within either less than 3 months (group A: average 7.3 days, 15 patients) or 6 and more months (group B: average 17 months, 8 patients) after the internal fixator had been removed. Quantitative and qualitative CT osteoabsorptiometry were used to assess changes in subchondral mineralization, reflecting the altered load acting on the adjacent and bridged zygapophysial joints. There was a significant difference between preoperative and postoperative calcium values (P < 0.001) for the whole patient group. Mineralization decrease was significantly more often found than increase (P < 0.001). A separate analysis of the 2 groups of patients revealed significant differences between group A and B (P < 0.001). In group A, a mineralization decrease was found in 61.3% and an increase in 11.0% of the facet joints, while in group B, a mineralization decrease was shown in 21.9% and an increase in 41.0%. No significant differences between adjacent and transfixed facets were found except in group B, in which the suprajacent joints showed a significantly higher mineralization increase than the transfixed joints (P = 0.030). Decrease in subchondral mineralization indicates reduced load acting on the examined zygapophysial joints. This finding in patients with early follow-up CT seems to be caused by reduced activity in most of the patients until removal of the spinal implant. In patients with longer intervals between removal of the fixator and second CT, higher loads acting on the adjacent and bridged joints are shown morphologically. Whether or not these changes lead to spondylarthritis has to be studied in a long-term follow-up.

Mechanical implications of humero-ulnar incongruity — finite element analysis and experiment

Previous studies show that the humero-ulnar joint is physiologically incongruous [Eckstein et al. (1995a) Anat. Rec. 243, 318–326] and exhibits a bicentric (ventro-dorsal) distribution of subchondral mineralization [Eckstein et al. (1995b) J. Orthop. Res. 13, 286-278]. We therefore asked: (1) Does humero-ulnar incongruity bring about a bicentric distribution of contact pressure? (2) Do tensile stresses occur in the subchondral bone of the trochlear notch that are in the same order of magnitude as the compressive stresses? (3) Do ventral and dorsal maxima of subchondral bone density correlate with a bicentric distribution of strain energy density? To that end, a two-dimensional finite element model was designed. The shape and material properties of the bones were based on CT and the boundary conditions selected to agree with resisted elbow extension at 90° of flexion. The incongruity and contact areas were determined experimentally from casts, and the pressure distribution with Fuji Prescale film. In the model and the experiment contact stresses above 2 MPa were recorded in the ventral and dorsal parts of the joint, and values below 0.5 MPa in the depth of the notch. In the model, tensile stresses of 2.9 MPa were observed in the subchondral bone of the ulna, but not in the humerus. The subchondral strain energy density yielded a bicentric pattern in a model with homogeneous subchondral bone properties. It is shown that humero-ulnar incongruity brings about a bicentric distribution of contact pressure, a tensile stress in the notch that is in the same order of magnitude as the compressive stress, and a distribution of strain energy density that correlates with subchondral density patterns.

Morphomechanics of the humero-ulnar joint: II. Concave incongruity determines the distribution of load and subchondral mineralization.

A deeper joint socket (concave incongruity) is found at most angles of flexion of the humero-ulnar joint and maintained over a wide range of physiological loading. It is, however, unclear how far this incongruity affects the distribution of load and subchondral mineralization of this joint as compared with a congruous configuration. Two nonlinear, axisymmetrical finite element models with two cartilage layers were constructed, one congruous and one incongruous, with a joint space of realistic magnitude. The distribution of subchondral mineralization was determined by computed tomography osteoabsorptiometry in the same six specimens that were investigated in the first part of the study, and compared with the biomechanical data obtained there and the predictions of the models. In the congruous case, the center of the socket is highly loaded, whereas the periphery does not experience mechanical stimulation. A central bone density maximum is predicted. With concave incongruity the position of the contact areas shifts from the joint margin towards the center as the load increases, and the peak stresses are considerably lower. A bicentric ventro-dorsal distribution pattern of subchondral mineralization is predicted, and this is actually found in the six specimens. Concave incongruity is shown to determine load transmission and subchondral mineralization of the humero-ulnar joint. It is suggested that this shape leads to a more even distribution of stress, provides intermittent stimulation of the cartilaginous tissue, and has beneficial effects on the metabolism, nutrition, and lubrication of the articular cartilage during cyclic loading.

Stress distribution in the trochlear notch. A model of bicentric load transmission through joints

In 16 cadaver humeroulnar joints, the distribution of subchondral mineralisation was assessed by CT osteoabsorptiometry and the position and size of the contact areas by polyether casting under loads of 10 N to 1280 N. Ulnas with separate olecranon and coronoid cartilaginous surfaces showed matching bicentric patterns of mineralisation. Under small loads there were separate contact areas on the olecranon and coronoid surfaces; these areas merged centrally as the load increased. They occupied as little as 9% of the total articular surface at 10 N and up to 73% at 1280 N. Ulnas with continuous cartilaginous surfaces also had density patterns with two maxima but those were less prominent, and in these specimens the separate contact areas merged at lower loads. The findings indicate a physiological incongruity of the articular surfaces which may serve to optimise the distribution of stress.

Kontaktflächen des menschlichen Humeroulnargelenks in Abhängigkeit von der Anpreßkraft, ihr Zusammenhang mit subchondraler Mineralisierung und Gelenkflächenmorphologie der Incisura trochlearis

Evaluation of the stress distribution in joints can be obtained directly from contact areas and pressure forces, and also indirectly from the functional adaptation of the connective tissues. Therefore 8 human humero-ulnar joints, fixed in formalin, were examined for size and position of contact areas (polyether casting/Vidas image analyser) and their dependence upon the joint forces (Zwick material testing machine). The distribution of subchondral mineralisation was assessed, using CT osteoabsorptiometry. Depending on the joint force, the contact areas increase from about 10% of the total surface (20 N) to approximately 60% (1280 N). With weak forces they are localised ventrally and dorsally in the joint, with more powerful forces they run together centrally. With a divided articular surface they join at about 160-640 N, with a continuous surface, at about 40-80 N. Divided joint surfaces show a bicentric mineralisation pattern of the subchondral bone with ventral and dorsal maxima. Continuous surfaces, on the other hand, usually show central maxima. Both the mineralisation pattern and the position of the contact areas suggest a physiological incongruity of the humero-ulnar joint surfaces, which vanishes with increasing pressure due to viscoelastic deformation of articular cartilage and subchondral bone. More marked incongruity is postulated for the divided surfaces than for the others. The consequent peripheral transmission of pressure seems to involve a functional principle, which, present in several human joints, leads to both optimal distribution of the stress and better nutrition of the articular cartilage.