17. Understanding proximal junctional kyphosis: New way of evaluating adjacent segment strain and vulnerability to failure after long segment instrumentation

Abstract

<h3>BACKGROUND CONTEXT</h3> Proximal junctional kyphosis (PJK) is a common and devastating complication of long segment fusions. Presumably, PJK occurs due to excessive or altered strain distribution at the proximal level above a fusion construct; however, this has not been previously studied or proven experimentally. <h3>PURPOSE</h3> Evaluate the patterns of proximal junction discoligamentous and bone strain distribution following long-segment pedicle screw fixation in a cadaveric model. <h3>STUDY DESIGN/SETTING</h3> Human cadaveric biomechanical testing. <h3>PATIENT SAMPLE</h3> Seven human cadaver specimens (T2-T12). <h3>OUTCOME MEASURES</h3> Range of motion (ROM), posterior rod strain (RS), as well as intervertebral disc (IVD), endplate and bone surface strains at the proximal junction. <h3>METHODS</h3> Nondestructive flexibility tests (4.0 Nm) were performed on 7 specimens (T2-T12) to compare range of motion (ROM), posterior rod strain (RS), as well as intervertebral disc (IVD), endplate and bone surface strains at the proximal junction during flexion, extension, axial rotation (AR) following pedicle screw fixation (PSR) from T4 to T12. Specimens were coated with a high contrast speckle pattern for optical tracking of movements using digital image correlation (DIC) with conversion to principle strain measured at the proximal junction disc and vertebral body bone surfaces. Data were analyzed using a two-way ANOVA followed by post hoc comparisons. <h3>RESULTS</h3> PSR resulted in significantly greater mobility at the proximal free level (T3-4) in flexion compared to intact (p=0.005), but not extension (p=0.3). PSR resulted in significantly lower ROM in flexion and extension across the instrumented T5-T12 levels (p<0.001) compared to intact. In flexion, PSR resulted in decreased strain at the posterior T3 inferior endplate compared to intact (p=0.038). In extension, PSR resulted in decreased strain at the anterior aspect of the disc (p=0.014), but increased strain at the posterior aspect of the disc (p=0.026) compared to intact. In AR, PSR resulted in elevated strain at the T3 inferior endplate (p=0.02), and reduced strain at the T4 superior endplate (p=0.028). Average T4 surface bone strain was lower with PSR compared to intact in flexion (p=0.039) and extension (p=0.028), and lower in the inferior anterior vertebral body in left AR (p=0.045). <h3>CONCLUSIONS</h3> This represents the first cadaveric biomechanical study of bone and soft tissue strain following long segment spinal instrumentation. Our results suggest that instrumentation alters the normal anterior-posterior and cranial-caudal strain profile across the proximal disc and vertebral body—the most common failure points in PJK. This study provides a foundation for understanding the biomechanics underlying PJK and will allow for the development of more targeted prevention strategies. <h3>FDA DEVICE/DRUG STATUS</h3> This abstract does not discuss or include any applicable devices or drugs.