Biomechanics of the mandible and growth extension.

BACKGROUND AND OBJECTIVE: One way of determining the direction of growth of the mandible is to consider the temporomandibular joint and movement of the mandible as a four-joint gear system, regarding growth then as an extension of the gear system. Our aim was to examine any correlations between the type of biomechanical growth extension and change in the maxilomandibular relation after Class II therapy. SUBJECTS AND METHODS: A total of 130 lateral cephalograms-before and after orthodontic treatment-were available from 65 adolescent class II patients with open bite or deep bite. The two lateral cephalograms from each patient were superimposed on the occlusal plane. Cephalometric values and the vertical base point deviation were determined from biomechanical analyses, together with three distances and three angles. RESULTS: No correlation between the cephalometric data and distances or angles were observed. Although there were no significant differences in the distances, we did note significant differences in all three angles (p < 0.05). CONCLUSION: If gear system extension during growth is considered, this can be interpreted as meaning that the occlusal plane of those patients with an initially open bite dropped during treatment, but that it rose in patients with an initially deep bite.

[Mid-term clinical and MRI results after refixation of osteochondral fractures with resorbable implants]. / Mittelfristige klinische und kernspintomografische Ergebnisse nach Refixation osteochondraler Fragmente mit resorbierbaren Implantaten.

AIM: Refixation of osteochondral fractures with resorbable implants is a common surgical treatment. There are almost no studies that prove good clinical outcomes. Hence, the aim of the study was to evaluate the mid-term results after refixation of osteochondral fractures. METHODS: The results of 12 patients were recorded 6.5 (±1) years after refixation of osteochondral fractures measuring 3.4 cm (2) (±2.5) of the knee (8 ×) or the ankle joint (4 ×) with resorbable inplants. Clinical scores and a modified MRI score based on that of Henderson et al. were used. RESULTS: The clinical scores showed good to excellent results after 6.5 (±1) years (VAS pain: 1.9 [±2.4], Tegner: 5.0 [±1.7], Lysholm: 84.8 [±14.3], McDermott: 91.3 [±7.9], Knee Society: 189.4 [±12.1]). MRI showed with one exception good integration of the fractures. In 3 cases subchondral cysts could be found. In 7 cases changes in the chondral outline occurred. The effect of this was a modified Henderson score of 12.6 (±3.7). The MRI results did not correlate with the clinical outcome. CONCLUSION: Because of its good clinical results the refixation with resorbable implants can be recommended to treat osteochondral fractures.

[A new total knee arthroplasty with physiologically shaped surfaces. Part 2: First clinical results]. / Eine neuartige Kniegelenksendoprothese mit physiologischer Gelenkform. Teil 2: Erste klinische Ergebnisse.

The human medial tibial plateau is concave, whereas the lateral tibial plateau is convex. In a normal knee, the convex femoral condyles roll and glide on the tibia during the standing phase of walking. The designs of most commercially available knee prostheses do not take this morphological feature into consideration. The novel design of the AEQUOS G1 knee replacement prosthesis is based on the natural anatomy of the knee joint, with a convex lateral tibia plateau and a sagittal offset of the medial and lateral compartments. Following extensive development and testing, initial clinical results of the AEQUOS G1 prosthesis in a mulitcenter study are presented. From Mai 2005 to March 2007, 158 patients in 4 clinics underwent total knee arthroplasty with the AEQUOS G1 and agreed to participate in the study. Patients were evaluated preoperatively and at 3, 6 and 12 months of follow-up using a standardized protocol that included the American Knee Society Score (AKSS), the Oxford Knee Score (OKS) and the Visual Analog Scale (VAS) for pain. After 3 months, 151 patients appeared for follow up appointments, after 6 months, 134, and after 12 months, 127. The mean range of motion preoperatively was 97.0 degrees (+/-19.9 degrees ) and 107.5 degrees (+/-15.9 degrees ) 12 months after surgery. The AKSS, as well as the modified OKS, significantly improved (p<0.0001) from preoperative scores of 98.8 (+/-35.8) and 37.3 (+/-6.9) points, respectively, to 165.8 (+/-34.1) and 21.9 (+/-7.8) points, preoperatively, and 12 months postoperatively. The VAS score significantly decreased (p<0.001) from 7.4 (+/-1.8) points preoperatively to 1.9 (+/-2.2) points 12 months postoperatively.One implant was revised because of arthrofibrosis and another due to patellar luxation. Two patients required revision because their implants revealed malalignement with ligamentous instability. No infections, aseptic loosening or other implant-specific complications were observed at this early follow-up. Good clinical results were observed at early follow-up with the AEQUOS G1 knee arthroplasty. However, longer follow-up is necessary for a general evaluation of the implant.

[A novel knee endoprosthesis with a physiological joint shape. Part 1: Biomechanical basics and tribological studies]. / Eine neuartige Knieendoprothese mit physiologischer Gelenkform. Teil 1: Biomechanische Grundlagen und tribologische Untersuchungen.

The natural tibiofemoral joint (TFJ) functions according to a roll-glide mechanism. In the stance phase (0-20 degrees flexion), the femur rolls backwards over the tibia plateau, while further flexion causes increased gliding. This kinematics is based on the principle of a quadruple joint. The four morphological axes of rotation are the midpoints of the curvatures of the medial and lateral femoral condyles and the medial and lateral tibia plateau. In addition, the medial and lateral compartments are shifted a few millimetres in a sagittal direction, the medial tibia plateau being concave and the lateral plateau convex. In most knee arthroplasties, these factors are not taken into account; instead they are equipped with symmetrical medial and lateral joint surfaces. Thereby, the midpoints of the curvatures of the sagittal contours of the lateral and medial joint surfaces, on the femoral as well as on the tibial sides, create a common axis of rotation which does not allow a physiological roll-glide mechanism. The goal of this study was therefore to report on the biomechanical basis of the natural knee and to describe the development of a novel knee endoprosthesis based on a mathematical model. The design of the structurally new knee joint endoprosthesis has, on the lateral side, a convex shape of the tibial joint surface in a sagittal cross section. Furthermore, from a mathematical point of view, this knee endoprosthesis possesses essential kinematic and static properties similar to those of a physiological TFJ. Within the framework of the authorization tests, the endoprosthesis was examined according to ISO/WC 14243 in a knee simulator. The abrasion rates were, thereby, lower than or at least as good as those for conventional endoprostheses. The presented data demonstrate a novel concept in knee arthroplasty, which still has to be clinically confirmed by long term results.

[Biomechanics of the tibiofemoral joint and knee braces]. / Zur Biomechanik des Tibiofemoralgelenks und deren Umsetzung bei Knieorthesen.

We investigated whether the kinematics of modern knee braces reflect the natural movement of the knee, especially with regard to the roll-glide ratio. Seven commercially available modern knee braces were analyzed in a new measuring unit with a 6-D positional registration system, which had been developed for this study. The results were compared to the theoretically postulated joint movements. All knee braces produced a roll-glide ratio different from the natural movement of the knee. Only the Townsend brace protected the anterior cruciate ligament at the beginning of the movement.Biomechanically, none of the knee braces tested provided efficient protection for the knee movement. We propose a new functional external joint, which allows restoration of the natural roll-glide mechanism of the articulating surfaces.

[An optical method for determining deformation and form changes in polyethylene surfaces of explanted acetabular cups of hip endoprostheses]. / Ein optisches Verfahren zur Bestimmung des Abriebs und der Gestaltsänderung der Polyethylenoberfläche explantierter Pfannen von Hüftendoprothesen.

AIM: Most methods used for the determination of volumetric wear of polyethylene cups are based on the assumption that the head of the prosthesis penetrates the cup in "cylindrical" fashion. The new accurate optical method is independent of this disputable assumption. METHOD: The articulating surface of the cup is scanned with light and a data set of 60,000 pixels obtained in this way is stored in a computer. Data obtained from used cups were compared with those obtained from unused cups. The volumetric wear was calculated directly by threefold integration. To assess the changes in surface shape, the data are fitted by an ellipsoid whose long axis defines the mean direction of load. A total of 18 retrieved and 3 unused cups of different types were studied. RESULTS: The unused acetabular cups deviated only slightly from ideal hemispheres. The surfaces showed rotational symmetry, and an undulation having an amplitude of 0.1 mm between dome and equator. For all explanted cups, the assumption of cylindrical penetration of the head into the polyethylene was shown not to represent the true situation. The cup expands in all directions, and the volumetric wear is underestimated by 50% with the traditional methods. The data suggest that long-term survival may be jeopardized when the main direction of loading is centered on the dome of the cup. Ceramic heads were associated with smaller rates of volumetric wear. CONCLUSION: The new optical method is characterised by short measuring times, precision and simple application. Analysis of the wear patterns of polyethylene components using this technique may contribute to a further understanding of the complex mechanisms of aseptic loosening.