[Biomechanical compensatory mechanisms of hips and spine : The essentials for spine and hip surgeons]. / Biomechanische Kompensationsmechanismen der Hüftgelenke und der Wirbelsäule : Das Wesentliche für Wirbelsäulen- und Hüftchirurgen.

Spinal alignment changes with age and degeneration. Different compensatory mechanisms of the spine are necessary to preserve spinal balance. The capacity of compensation of the spine decreases with age. Thus, the pelvis and the lower limbs become involved in the compensatory mechanism. Concomitant osteoarthritis of the hip could impair this capacity. The biomechanical principles of compensation are described with respect to planning reconstructive hip and spine surgery.

Left-right axial rotation within C1-2 after implant removal.

OBJECT: Surgical treatment of atlantoaxial injuries may be performed by a variety of surgical procedures, with each of these having its own specific advantages and disadvantages. To preserve the range of motion within the atlantoaxial joint after surgical treatment, posterior atlantoaxial screw fixation according to the method of Goel and Harms could be beneficial. This technique is not considered to fix the joint permanently if the screws are removed. However, this must not necessarily be true, especially if one notices that cervical joints have a tendency for rapid fusion after surgery. The objective of this study was to analyze left-right axial rotation following implant removal, with the following research questions addressed: 1) is there a relevant rotational left-right mobility in C1-2; 2) is there a difference in mobility depending on sex; 3) is there a correlation of mobility to age; 4) is there a correlation of mobility to the "implant-in-body time"; and 5) is neck pain improved by this method of surgical treatment? METHODS: This is a retrospective study in 10 patients who had received atlantoaxial fixation according to the Harms method following atlantoaxial injury. These patients had undergone implant removal after a mean time of 128 days, followed by functional atlantoaxial CT in left-right rotation. Statistical analysis was performed using the SPSS software package; significance was assumed for p < 0.05. RESULTS: The following findings are reported. 1) Left-right axial rotation within the atlantoaxial joint was measured to be 17° (mean value) following implant removal. 2) There was no significant difference between men (20°) and women (14.8°) (p = 0.595). 3) Left-right axial rotation negatively correlated to the age of the patient (r = -0.646, p = 0.043). 4) "Implant-in-body time" did not influence left-right axial rotation (r = 0.04, p = 0.907) if the implants are removed within 90-180 days after surgery. 5) Neck pain as assessed by the patients themselves on the visual analog scale was 7.9 ± 1.1 preoperatively and 3.6 ± 1.2 (mean ± SD) after implant removal (p = 0.002). CONCLUSIONS: The authors conclude that removal of the implants 3-6 months after posterior atlantoaxial fixation according to the method of Goel and Harms is beneficial for obtaining some axial rotation within the atlantoaxial joint. The range of motion preserved depends on the age of the patient.