Full length articleThe effect of positive sagittal spine balance and reconstruction surgery on standing balance
Introduction
Sagittal spinal alignment consists of lumbar lordosis, thoracic kyphosis, and cervical lordosis. When optimal sagittal alignment exists, the horizontal distance in the sagittal plane between the center of the C7 vertebra and the posterior superior aspect of the S1 vertebra is less than 5 cm measured on x-ray using the plumb-line method [[1], [2], [3]]. In individuals with positive sagittal spine balance (PSSB), the C7 vertebra lies more than 4 cm anterior to the S1 vertebra [2]. Degenerative spine conditions such as flatback syndrome, degenerative kyphosis, and disc disease can lead to PSSB [2]. PSSB results in anterior displacement of the trunk making it appear as though the person is leaning forward. PSSB increases vertebral intradiscal pressure, spinal loads, and the work of spine muscles to counter trunk forward displacement or lean [[4], [5]]. Consequently, individuals with PSSB experience spine muscle fatigue and back pain and have difficulty maintaining a horizontal gaze while standing and walking [[4], [5], [6], [7]].
Anterior displacement or lean of the trunk can move the center of mass (COM) of the whole body and hence the center of pressure (COP) out of the base of support (BOS) formed by the perimeters of the feet [[8], [9]]. Fall risk and loss of balance may increase when COM/COP lies outside the BOS [10]. Individuals with PSSB often recruit compensatory mechanisms to achieve a more upright posture and bring the COM into the BOS [[9], [11]]. The most common compensatory mechanisms observed in individuals with PSSB are hyperextension of intervertebral joints and increased posterior pelvic tilt, hip flexion, knee flexion and/or ankle plantar flexion while standing [[12], [13], [14]].
Compensatory mechanisms for anterior displacement of trunk COM have been explored in able-bodied subjects while standing and walking [[9], [15]]. When asked to stand with varying amounts of trunk flexion, able-bodied subjects increased hip flexion and ankle plantar flexion to bring their COM/COP within the BOS; these compensatory mechanisms were associated with increased metabolic energy expenditure [9]. Similarly, when asked to walk with varying amounts of trunk flexion, able-bodied subjects adopted a crouch gait (i.e., increased mid-terminal stance knee flexion) [15]. Similar increases in hip and/or knee flexion during walking have been observed in individuals with flat back syndrome [14] and in a more heterogeneous group of persons with degenerative spine disease [16] to compensate for PSSB. These compensatory mechanisms often increase the work of lower limb muscles, thus contributing to lower limb muscle fatigue [7]. Muscle fatigue in the lower limbs and trunk has been shown to increase postural sway and negatively affect standing balance in various populations [[17], [18]].
PSSB can be corrected by spinal reconstructive surgery [16]. The primary goal of surgery is avoidance of further spinal degeneration and pain by optimizing craniospinopelvic alignment. As a result, reconstructive surgeries have been reported to improve function and quality of life [[7], [19]]. Post-operatively, individuals are able to maintain a horizontal gaze and may have a sense of improved balance. However, standing balance has not been quantified in individuals with PSSB. Standing balance can be quantified by measuring postural sway: a measure of COP amplitude in the plane of the BOS [[20], [21]]. If the COP lies close to the boundary of the BOS, a small perturbation can push it outside the BOS, increasing fall risk [[22], [23]]. Balance is considered poor if COP amplitude is greater than normal limits [24].
While using the plumb-line method on an x-ray [2] can provide a measure of spinal balance in individuals with PSSB pre- and post-operatively, it cannot provide a measure of the effects of PSSB on standing balance before and after surgical intervention. This study attempted to quantify standing balance using postural sway measures before and after reconstructive surgery in persons with PSSB. Our primary hypotheses were that (1) pre-operative postural sway values would be larger than those of able-bodied individuals and positively correlated to the degree of PSSB and (2) that postural sway would improve after surgical correction of PSSB, moving closer to that of able-bodied individuals. Also, since a previous study reported that crouch gait resolved after reconstructive surgery in individuals with PSSB [16], we wanted to assess whether similar resolution of crouch posture was achieved in this study. Hence, our secondary hypothesis was that hip flexion, knee flexion, and ankle dorsiflexion (all of which are greater than normal in crouch gait) would decrease after reconstructive surgery in individuals with PSSB during standing.
Section snippets
Methods
Subjects participated in this Institutional Review Board (IRB) approved study after providing informed consent. Subjects met the following inclusion criteria in order to participate: 1) 18–80 years of age, 2) PSSB of more than 7 cm as measured using the plumb-line method on x-ray [2], 3) planned reconstructive spine surgery to correct PSSB, and 4) ability to stand on level ground unaided for a short period of time. Exclusion criteria included: 1) morbid obesity (in particular the presence of a
Results
Sixteen subjects with PSSB were enrolled in the study; completed data sets at all three time points were available for nine subjects. One subject was excluded entirely due to pelvic marker problems caused by a pendulous abdomen. Of the nine subjects who completed data collection at all three time points, one subject presented with severe hip pain at 6–12 months post-surgery and another subject presented with a broken ankle at 24 months post-surgery, so data from these time points were excluded
Discussion
This study investigated the effects of PSSB and spinal reconstructive surgery on postural sway in individuals with PSSB. The study also investigated the effect of spinal reconstructive surgery on hip, knee and ankle (dorsi) flexion in individuals with PSSB. As expected, the degree of PSSB decreased in all the subjects post-operatively. The pre-operative normCOPap and normCOPml of individuals with PSSB were greater than those of able-bodied individuals, with normCOPap moderately correlated to
Conclusions
PSSB in persons with degenerative spinal conditions compromised postural balance as indicated by greater than normal postural sway; this improved significantly when PSSB was corrected by spinal reconstructive surgery. Additionally, persons with PSSB adopted crouch posture as evidenced by increased hip and knee flexion; this resolved after spinal reconstructive surgery.
Conflict of interest statement
All authors have no conflict of interest.
Acknowledgements
Funding for this project was provided by Medtronic Sofamer-Danek Inc. The authors acknowledge the use of the Jesse Brown VA Medical Center Motion Analysis Research Laboratory, Tyler Koski, MD, and Kristen Smith, RN, for assistance with funding and subject recruitment.
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