Monday, September 1, 2008

To What Degree Does Posterior Scoliosis Instrumentation and Fusion Limit Growth?

Chris Moore was a Summer Intern over the past few months at Hey Clinic, and did an awesome job!   He is a mechanical engineer working in healthcare product development, and is in the process of completing his coursework to enter into a full-time healthcare career.  He did an outstanding job this summer, helping out with our biomechanical research on better scoliosis instrumentation techniques, and also spent a lot of time with us in the operating room and in the clinic.  He also wrote up the following paper in response to several questions we received from parents of scoliosis patients who were considering surgery.  Chris’ summary of the literature helped many parents and adolescent scoliosis patients feel much more comfortable that they would not lose a lot of growth in most cases of spinal fusions.  While many families and some consulting doctors do bring up the issue of concerns about “stunted growth” from scoliosis or kyphosis fusion surgery, virtually all families and patients are very pleasantly surprised at their child’s new improved height and posture after surgery. I have not had any patients or families with any concerns about short stature or short trunk years down the road after fusion.

Great Job, Chris, and best of luck as you complete your course work, and pursue a career as a “hands on” healthcare provider.  You will be great!

Lloyd A. Hey, MD MS
Hey Clinic for Scoliosis and Spine Surgery.
Raleigh, NC  USA

To What Degree Does Posterior Scoliosis Instrumentation and Fusion Limit Growth?

By  W Chris Moore, Lloyd A. Hey, MD MS
September 1, 2008
Hey Clinic for Scoliosis and Spine Surgery
Raleigh, NC  USA

Adolescent Idiopathic Scoliosis (AIS) is characterized by an abnormal curvature of the spine brought on by undetermined causes.  While much research has been dedicated to proving causality for AIS, results remain inconclusive.  One fact that has been proven is the greatest progression of the spinal deformity (mostly measured using the Cobb angle) occurs during periods of peak growth in the vertebral body.  In fact, spinal development is not occurring at a constant rate and it actually fluctuates over a human’s infantile, juvenile, and adolescent periods.1 Therefore, understanding when the greatest growth velocity occurs is important for physicians determining how to best treat a patient.  Knowing that scoliosis often progresses after reaching a Cobb angle of 40 degrees  , scoliosis surgeons will  usually recommend correcting a patient’s scoliosis curve through posterior hardware fusion. Due to the large amount of growth remaining in children under age 6-8 years of age, posterior fusion surgery alone is often avoided, since the continued anterior growth of the spine could result in new deformity called “crankshaft phenomenon.”  To avoid this problem in the younger child, surgery may be delayed, or “growing rods” inserted temporarily, or anterior/posterior fusion is performed to stop the anterior growth and subsequent possible crankshaft phenomenon  (rare).  Crankshaft phenomenon does not appear to be an issue with posterior fusion in children age 8 or above in most cases, since the amount of anterior growth along the discs is less.  In this article, we will be focusing on the adolescent population, rather than this younger group, under age 6-8.  For many adolescent scoliosis patients and their parents,  understanding the potential impact of spinal fusion  can have on their remaining growth can be important as they consider timing of scoliosis or kyphosis surgery.
A common scoliosis correction, such as a T5-L1 fusion, will encompass a total of eight thoracic vertebrae, eight thoracic discs, and one lumbar vertebra.  Determining the inhibited growth of a patient with this type of fusion will require knowledge of the growth rates of each type of vertebrae and discs included. Growth rates of vertebrae are dependent on a patient’s age and sex. Although, according to Dimeglio and Ferran’s study2, the longitudinal growth rate of thoracic vertebrae is 0.8mm per year on average, while the lumbar vertebrae average 1.1mm per year.  Roaf3 and Taylor4 estimate thoracic intervertebral discs have average growth rates between 0.2 and 0.6mm per year and lumbar disc growth rates of 0.3 to 0.8mm per year.  Using these figures as an estimate of annual growth rate, a fusion from T5-L1 would appear to restrict 11.1mm (0.44 in) of growth per year.  However, this calculation can not be assumed correct for each year of a patient’s development since the rate of growth is, as mentioned previously, not consistent throughout adolescence. In fact, according to studies published by Wever et al5, who measured spinal growth from T1 to L4 of 60 patients, the maximum growth velocity of a female’s spine occurred between the ages of 11.5 and 12.5 years, and for the six male patients included in the study, the maximum growth velocity occurred at the age of 15.  At these times, the mean maximum growth velocity for females was 18.3mm per year and 26.1 mm per year from T1 to L4.  By comparison, if we calculate the growth of this section of spine using Dimeglio, Ferran, Roaf, and Taylor’s average growth rates, the growth is determined to be 21.05 mm per year which is within the range of Wever’s estimated growth. This shows some consistency between the two estimation methods.
Therefore, to accurately deduce how much growth will be impeded by a T5 – L1 fusion, one must consider how much growth remains when they have the correction surgery.  Several charts from Wever’s journal article reveal average growth curves for adolescent females which is useful for determining this amount.  Figure 1 provides useful data about the overall growth rate of T1-L4 per year from ages 9 through 18 of the females studied.
Using Dimeglio, Ferran, Roaf, and Taylor’s growth rates merely as a ratio of growth for thoracic vertebrae, thoracic discs, lumbar vertebrae and lumbar discs; it is possible to extrapolate each vertebral body’s growth rate per year.
The growth tables shown in Table 1 were generated using the Wever (ref 5)  growth data in Figure 1 in conjunction with growth ratios from Dimeglio and Ferran2, Roaf3, and Taylor4.
So what can this data provide?  For a female patient, the length of their fusion and their current age can predict how much remaining growth is restricted.  Simply multiply the number of discs/vertebrae included in the fusion by their respective remaining growth values and sum up the values.

Example T5-L1 Fusion:  (See Figure 2 with Spine Diagram.)
A female having this procedure at age 9 would expect 56.8 mm (2.2in) of growth inhibited by the fusion; however, a female at age 14 would expect 17.7mm (0.7in) of inhibited growth for the fusion region.  Once again these are estimates of averages and can only be applied to female patients. While Weaver (ref 5) does provide growth data on male development, male growth curves were not generated because the sample size was too small (only six males were followed in the case study). Additional searches for male growth data were unsuccessful and must be obtained before estimating similar fusion implications.  Once again, Weaver5 does state that among the six males studied, the maximum growth velocity of the spine occurred at age 15 compared with the females maximum growth velocity occurring at 11.5–12.5.  Therefore it is reasonable to believe the remaining growth of adolescent boys would be greater than for females of the same age. Those seeking to understand spinal development better would be benefited by conducting a new study which tracks the growth rates of males.  Also, the inhibited growth presented in this document does not take into account the additional growth that may continue to occur once the hardware is implanted. Searches for this information were also conducted but were unsuccessful. Lastly, it is important to remember that straightening a patient’s spine with a severe scoliosis curve will inherently provide “growth” by instantaneously adding inches to a patient through the straightening their curve, which could add anywhere from a half an inch up to over 3 inches in height in severe kyphosis or double scoliosis surgery corrections.


Sarwark, J. and C. E. Aubin (2007). "Growth considerations of the immature spine." J Bone Joint Surg Am 89 Suppl 1: 8-13.

Dimeglio, A. and J. L. Ferran (1990). "[Three-dimensional analysis of the hip during growth]." Acta Orthop Belg 56(1 Pt A): 111-4.

Roaf, R. (1960). "Vertebral growth and its mechanical control." J Bone Joint Surg Br 42-B: 40-59.
4. Taylor, J. R. (1975). "Growth of human intervertebral discs and vertebral bodies." J Anat 120(Pt 1): 49-68.
5. Wever, D. J., K. A. Tonseth, et al. (2002). "Curve progression and spinal growth in brace treated idiopathic scoliosis." Stud Health Technol Inform 91: 387-92.

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