Concept of variable angle locking—evolution and mechanical evaluation of a recent technology

Applications for fracture‐adapted screw positioning offered by variable angle locking screws are increasing. The locking strength of the variable angle locking mechanism at different insertion angles was compared to conventional fixed angle locking screws. Stainless steel (S) and titanium (Ti) variable and fixed angle 2.4 mm locking screws, inserted at different inclinations (0°?15°), and locked at 0.8 Nm were subjected to a load‐to‐failure test. Ultimate failure moment at the screw‐head interface and failure mode of the screws were determined. Significant differences were detected by one‐way ANOVA (p < 0.05). Stainless steel and titanium variable angle locking screws inserted at 2°?10° inclination exhibited a failure moment comparable to the 0° position (S 1.67± 0.04 Nm; Ti 1.67 ± 0.14 Nm) and failed predominantly at the screw neck, with the head remaining fixed to the plate. Inserted at 15° inclination, screws revealed a lower failure moment (S 1.33 ± 0.06 Nm, Ti 1.58 ± 0.05Nm), and failed predominantly by breakout of the head thread. Fixed angle locking screws inserted at an inclination >2° did not lock properly in the plate hole, providing insufficient locking strength. Variable angle locking screws offer a stable head‐locking mechanism at different inclinations, comparable to the locking strength of orthogonal inserted fixed angle locking screws. Marginal inclinations >15° should be used with care. © 2015 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 33:988–992, 2015.

     

多维度评估对特发性脊柱侧凸进展高峰期的预测价值

目的:通过多维度评估特发性脊柱侧凸(idiopathic scoliosis,IS)患儿侧凸进展高峰期(peak angle velocity,PAV)的相应成熟度指标,探讨其对PAV的预测价值。方法:选取初诊时月经未至、Risser征0级,Y三角软骨未闭、以半年为周期随访至少5次且进展超过5°的女性IS患儿30例,初诊年龄为10.8±1.5岁(7.8~12.1岁)。随访时收集及测量的指标包括实足年龄、Y三角软骨闭合状态、Risser征、身高、主弯Cobb角和骨龄(digital skeletal age,DSA)评分。计算每个随访周期内的身高增长速度(height velocity,HV)和Cobb角增长速度(angle velocity,AV)。采用逻辑回归分析各维度指标对PAV的预测价值。结果:30例患儿的PAV年龄为11.8±1.3岁(10~15岁),PAV时的身高为150.7±4.8cm,DSA评分为479.4±56.5,主弯Cobb角为26.5°±7.2°,HV为8.3±3.7cm/年,AV为7.8°±5.2°/年。PAV主要发生于Risser征0级(80.0%)和1级(20.0%)。逻辑回归分析结果显示PAV的发生与实足年龄介于11~13岁之间(OR=3.166,P=0.032)、Y软骨闭合(OR=6.365,P0.01)、Risser征0级(OR=12.963,P0.01)、DSA评分介于400~500之间(OR=10.758,P=0.011)、HV6cm/年(OR=4.346,P=0.025)、Cobb角30°(OR=9.535,P=0.013)呈显著相关。结论:年龄介于11~13岁之间、Risser征0级伴Y软骨闭合、DSA评分介于400~500之间、HV6cm/年和Cobb角30°为女性IS患儿侧凸PAV的高危因素,可用于预测PAV的发生,评估侧凸进展风险。