Narrative Review of Clinical Impact of Head-Hip Offset Following Adult Spinal Deformity Surgery

Abstract

In adult spinal deformity (ASD) surgery, mechanical failure (MF) has been a significant concern for spine surgeons as well as patients. Despite earnest endeavors to prevent MF, the absence of a definitive consensus persists, owing to the intricate interplay of multifarious factors associated with this complication. Previous approaches centered around global spinal alignment have yielded limited success in entirely forestalling MF. These methodologies, albeit valuable, exhibited limitations by neglecting to encompass global balance and compensatory mechanisms within their purview. In response to this concern, an in-depth comprehension of global balance and compensatory mechanisms emerges as imperative. In this discourse, the center of gravity and the gravity line are gaining attention in recent investigations pertaining to global balance. This narrative review aims to provide an overview of the global balance and a comprehensive understanding of related concepts and knowledge. Moreover, it delves into the clinical ramifications of the contemporary optimal correction paradigm to furnish an encompassing understanding of global balance and the current optimal correction strategies within the context of ASD surgery. By doing so, it endeavors to furnish spine surgeons with a guiding compass, enriching their decision-making process as they navigate the intricate terrain of ASD surgical interventions.

Keywords: Adult spinal deformity; Compensatory mechanism; Global balance; Global spinal alignment; Head-hip offset; Mechanical failure.

Fig. 1.

Normative offset distance between bony landmarks and the gravity line [12]. Positive values signify anterior to the gravity line and negative values signify posterior to the gravity line. CAM : center of acoustic meatus, CI : confidence interval.

Fig. 2.

GL-HA offset [1]. Projection of the GL (orange line) and HA (blue dashed line) on a whole-body sagittal radiograph. A negative GL-HA offset value indicates that the GL is posterior to the HA, and vice versa for a positive value. The GL-HA offset is -28.3 mm in the presented case.

Fig. 3.

Immediate postoperative appropriate GL-HA offset having no mechanical failure. A 65-year-old female (F), PSSIF T9 to pelvis with multilevel posterior column osteotomy, including TLIF at levels L2/3, L3/4, L4/5, and L5/S1. A : The preoperative GL-HA offset of 196.6 mm was appropriately corrected to -15.5 mm postoperatively. It did not exceed the risk factor of -50 mm, which is associated with mechanical failure. B : The GL-HA value has been well maintained at -10.0 mm for 2 years postoperatively, and there have been no significant mechanical or clinical changes up to the present. PSSIF : posterior spinal segmental instrumentation and fusion, SVA : sagittal vertical axis, LL : lumbar lordosis, GL : gravity line, HA : hip axis, PI : pelvic incidence, preOP : preoperative, PO : postoperative.

Fig. 4.

Immediate postoperative inappropriate GL-HA offset with mechanical failure. A 70-year-old female (F), PSSIF T10 to pelvis and uninstrumented posterior fusion T9-10, prophylactic vertebroplasty (VP) T9, T10, therapeutic VP at L2 including decompressive laminectomy L1/2/3, bilateral foraminotomy at L5-S1. A : The preoperative GL-HA offset of 5.5 mm was excessively corrected to -128.9 mm postoperatively. It did exceed the risk factor of -50 mm, which is associated with mechanical failure. B : The GL-HA value has been changed at 11.0 mm for 3 years postoperatively, and it have resulted in bilateral rod fracture (red arrows) and symptoms of back pain. GL : gravity line, PSSIF : posterior spinal segmental instrumentation and fusion, PL : plumb line, LL : lumbar lordosis, PI : pelvic incidence, SVA : sagittal vertical axis, HA : hip axis, preOP : preoperative, PO : postoperative.

Fig. 5.

Cranial sagittal vertical axis to the centers of the hip [18]. CrSVA-H (blue line) : the distance between the vertical line (yellow line) of the CCOM and the center of the two femoral heads. Nasion-inion line (yellow dashed line) : root of the nose to the external occipital protuberance, CCOM (midpoint of the nasion-inion line) : above and slightly in front of the acoustic meatus. CrSVA-H : cranial sagittal vertical axis to the centers of the hip, CCOM : cranial center of mass.