Biomechanical stability of lower cervical spine immediately after discectomy with grafting

Abstract

Objective:  Anterior cervical discectomy is commonly used to treat radiculopathy and myelopathy. Although the size of the implanted graft may influence the clinical outcome of anterior reconstruction of the cervical spine, the ideal graft height remains arguable. The objective of the current study was to study the interrelations of graft height and immediate biomechanical stability in an anterior cervical discectomy model.

Methods:  Six fresh‐frozen human cadaver cervical spines (C1–T1) were tested in five sequential states. The first state tested was the “normal” state (specimens with intact discs). The other four states were tested after C5–C6 discectomy by the Smith‐Robinson graft technique, using graft thicknesses of 100%, 120%, 140%, and 160% of the baseline height. The baseline height was defined as the intervertebral disc height of C5–C6 at the intact stage. Intervertebral segment flexion, extension, bending and rotation of C5–C6 were recorded using a 3D laser scanner and analyzed using Geomagic Studio 5.0 software.

Results:  Bone grafting at 100% baseline height after discectomy provided the least stability and the greatest movement range. With increasing height of grafts, the movement range of the cervical spine declined. Immediate stability of the operated segments was significantly increased by grafting with 140% and 160% baseline heights compared to the baseline height condition.

Conclusions:  Strut‐graft with appropriate distraction after Smith‐Robinson anterior cervical discectomy plays an important role in the whole immediate biomechanical stability of the lower cervical spine. A graft height of 40% greater than baseline may be ideal after single discectomy in clinical practice.

Introduction

Cervical spondylosis is a common cause of radiculopathy and myelopathy due to extrinsic neural compression, especially in the elderly. Anterior cervical discectomy techniques have been widely used to treat radiculopathy and myelopathy ¹ . Good to excellent outcomes have been reported in 73% to 96% of all patients ² . However, these procedures may be complicated by graft collapse or extrusion, endplate failure, or nonunion ³ .

The size of the implanted graft may affect the clinical outcome. Historically, Robinson and Smith recommended the use of 10 to 15 mm grafts ⁴ . More recent studies have suggested smaller grafts of 4 to 7 mm in size ⁵ . However, some researchers have favored between 1 and 2 mm of distraction, without biomechanical justification ⁶ , ⁷ , ⁸ . An et al. performed a cadaver radiographic analysis and determined that, for a preoperative disc height of 3.5–6.0 mm, an interbody graft of 2 mm above the baseline thickness was most appropriate ⁹ . However, the influences of the graft height on immediate biomechanical stability have not been properly characterized. The objective of the current study was to study the interrelations of graft height and immediate biomechanical stability in an anterior cervical discectomy model.

Materials and methods

Specimens

The experimental protocol was approved by the institutional Human Investigation Committee. Six fresh‐frozen human cadaver cervical spines (C1–T1) were obtained from the Department of Anatomy, Nanfang Medical University. All the subjects were victims of acute cranial injuries, and had no history of orthopedic disorders. They died at the ages of 38–53 years (average, 44.6 ± 5.3 years), with heights of 165–180 cms (average, 173 ± 4 cms) and weights of 63–83 kgs (average, 71.6 ± 6.2 kgs). Plain X‐ray films showed evidence of lower grade spondylosis in all six cadavers. Cervical spines from C1–T1 were retrieved for the experiment. Soft tissues other than the joints, ligaments, articular capsules and longus colli were removed. The specimens were obtained 3.5 to 5.0 hs (average, 4.15 ± 0.78 hs) after death, double‐sealed and preserved at −20°C for 15 to 25 days (average, 21.36 ± 3.87 days) until testing. Roentgenograms were performed before sampling to exclude abnormalities and to determine the size of the grafts. The specimens were thawed at 4°C for ten hours before the experimental procedure. Both ends of each spinal sample (C1–C2 and C7–T1) were embedded in polymethyl methacrylate ¹⁰ .

Experimental model

Each specimen was tested in five sequential states. The first state tested was the “normal” state (specimens with intact intervertebral discs) and the remaining four states were tested after C5–C6 discectomy and Smith‐Robinson grafting ⁴ using four different sized grafts, namely 100%, 120%, 140%, and 160% of the “baseline height”. The baseline height of C5–C6 intervertebral space was determined from a plain anteroposterior X‐ray film before discectomy by measuring the distance between the anterior edges of the vertebrae. The “U” shaped grafts were made from iliac crest bone and had a width of 12 mms and depth of 9 mms.

Measurement

Standard discectomy at C5–C6 and Smith‐Robinson grafting were performed. Two Strut nails were inserted into the middle of C5 and C6, and a Caspar retractor utilized to distract the cervical vertebral bodies to allow correct insertion of the bone graft (Fig. 1). With a marker inserted into each of C5 and C6, the spine was fixed on a Material Test System (MTS, 858 Mini Bionix, MTX, Tokyo, Japan) located at the Biomechanics Center of Nanfang Medical University. Pure moments were applied in flexion‐extension, lateral bending, and rotation within physiologic limits (= 1.5 Nm). The specimen was humidified throughout the testing period. Movement of the C5–C6 intervertebral segment in flexion, extension, lateral bending and rotation was recorded by a 3D laser scanner (Optix 400, 3D Digital Corporation, Sandy Hook, CT, USA) with Realscan USB software. The markers were 3D reconstituted (Fig. 2) and the degrees of movement measured for analysis with Geomagic Studio 5.0 software (Geomagic, Research Triangle Park, NC, USA).

Figure 1.

Photo of the apparatus used for testing the range of cervical spine movement. A Caspar retractor was attached to the two Strut nails to distract the C5–C6 cervical vertebral bodies so that an appropriate bone graft could be inserted.

Figure 2.

The markers were reconstituted in 3D and the degrees of movement measured with Geomagic Studio 5.0 software.

Statistical analysis

Two testing cycles per specimen per state were performed and the two readings averaged as the final data for analysis. One‐way ANOVA was employed to compare the intervertebral movement data among the five states using the Statistical Product & Service Solution 13.0 (SPSS, Chicago, IL, USA). A post‐hoc Fisher's least significant difference test was used to determine the statistical significance of the differences in intervertebral segment movement between the various states. Data was expressed as the mean ± standard deviation and a P‐value of less than 0.05 was determined as a statistically significant difference.

Results

According to gross inspection and roentgenography, there was no bone fraction or collapse of vertebrae, end‐plates and grafts during the experiment. Annular ligament laceration occurred in one of the specimens with 160% of baseline height grafting. Minor graft position shifting occurred in another specimen while measuring the movements of flexion and extension at 100% of baseline height; justification and re‐measurement was performed for data validation. No graft extrusion was noticed in any of the specimens.

The ranges of movement at C5–C6 in the five testing states were digitized, captured and analyzed. The original readings from the duplicate tests on each specimen per test setting were very close, indicating consistency of the data. Bone grafting at 100% baseline height after discectomy provided the least stability and the greatest movement range. With increasing height of grafts, the movement range of the cervical spine declined (3, 4, 5). Figure 3 illustrates the average degrees of flexion and extension of the C5–C6 samples following implantation of grafts of different heights. The sample which had undergone baseline height implantation had an average of 15° movement in flexion and extension whereas the mean movement ranges of spines with 140% or 160% baseline height grafts were only 8.8° and 6.3°, thus the greater graft heights conferred significantly greater stability on the cervical spine (Fig. 3). Although this difference was not statistically significant, in the normal intact state the specimens flexed/extended markedly less than the samples with 100% baseline height grafting, but more than after 140% or 160% baseline height implantations. The results of lateral bending of C5–C6 revealed an almost linear negative correlation between the different heights of graft implantations (Fig. 4). However, rotation movement in the various implantation states did not noticeably vary (Fig. 5).

Figure 3.

Summary of extent of spinal flexion/extension following implantation of various heights of graft (n= 6).

Figure 4.

Summary of extent of spinal lateral bending following implantation of various heights of bone grafts (n= 6).

Figure 5.

Summary of extent of spinal rotation following implantation of various heights of bone graft (n= 6).

For statistical analysis, we compared the mean differences between testing groups as shown in Table 1. No tested specimen showed any statistical difference in ranges of movement compared with the normal intact state with one exception, namely the 160% baseline graft implantation significantly limited lateral bending of the spine (P < 0.05). However, thicker graft implantations (140% and 160% baseline height) after discectomy significantly restricted the cervical spines from flexion/ extension and lateral bending, when compared with the baseline height grafting condition (P < 0.01).

Table 1.

Interclass multiple comparisons on the range of movement

Group Comparison	Flexion and Extension		Lateral Bending		Rotation
	MD	P value	MD	P value	MD	P value
Normal : baseline	3.875	0.170	2.595	0.098	1.895	0.369
Normal : 120% ↑	0.352	0.899	0.353	0.817	0.202	0.923
Normal : 140% ↑	2.597	0.352	2.193	0.158	1.488	0.479
Normal : 160% ↑	4.632	0.103	3.425	0.032*	2.213	0.295
Baseline : 120% ↑	4.223	0.135	2.242	0.150	2.097	0.321
Baseline : 140% ↑	6.472	0.026*	4.788	0.004**	3.383	0.115
Baseline : 160% ↑	8.507	0.005**	6.020	0.001**	4.108	0.058

^* P < 0.05 and

^** P < 0.01, levels of statistically significant difference.

MD, mean difference between groups.

Discussion

Based on published reports and the authors' clinical experience, suitable strut‐graft implantation is commonly suggested following cervical discectomy. It is exceptionally beneficial for maintaining intact height and curvature of the cervical spine, enhancing stability of the relevant segments, securing a good remission rate and minimizing complications ¹¹ , ¹² , ¹³ . Over‐distraction of the cervical spine caused by excessively high grafting may lead to greater longitudinal stress and subsequently increase the incidence of complications such as collapse of the graft and fracture of the end‐plate ¹⁴ , ¹⁵ . On the other hand, inadequate distraction may result in prolapse of the graft ¹⁶ . Although the height of the graft appears critical to the clinical outcome and prognosis, the optimal height is still in question.

Much published research has focused on the influence of graft height on the size of the intervertebral foramen, trend towards non‐union, and stress between end‐plate and graft ⁹ , ¹¹ , ¹⁴ , ¹⁵ , ¹⁷ . The stability of the operated segments is also an important index in evaluating the quality of the surgery ¹⁸ . However, the biomechanical stability of the lower cervical spine immediately after discectomy and strut‐grafting has not been conclusively studied. In the current study, we evaluated the correlations between graft height and immediate stability of the lower cervical spine by measuring the movements in 3D. We postulated that the range of movement would correlate negatively with the stability of the strut‐grafted spines. To minimize the effects of individual variation, the height of the C5–C6 disk space of each sample was measured in the intact condition and referred as the “baseline height”. All subsequent test grafts were expressed as a percentage of the baseline heights rather than as absolute lengths.

Most published in vitro biomechanical investigations have focused on the internal system of vertebrae, discs and ligaments. Inclusion of the surrounding muscles, tendons and nerves adds extreme technical difficulties. In our specimens we preserved the longus colli muscles, along with the posterior longitudinal and annular ligaments, in an attempt to maintain some external stabilizing system. Standard surgical procedures were performed to avoid tearing the muscles/ligaments and uneven distraction to the frontal part of the vertebra during grafting. An alternative possibility is that external immobilization and anterior plating fixation could be performed in the clinical situation to preserve the initial cervical stability, yet appropriately sized grafts of optimal height can maintain the internal stability of the cervical spine and promote local bone healing. Moreover, the latter procedure may make internal plate fixation unnecessary and shorten patient recovery time.

The data from this study clearly show a positive correlation between stability of the involved segments and increasing graft height. As predicted, grafting at the baseline height showed less stability than the intact cervical spine. Even at 120% of baseline height, stability of lateral bending was unacceptably poor. Apparently loss of the intervertebral connection of discs and anterior longitudinal ligaments after discectomy dramatically destabilizes the cervical spine. However, 140% and 160% baseline height grafts provided much better stability. The data indicate that immediate stability of the cervical spine after discectomy can be restored, even increased, by suitable strut grafting.

It is also important to avoid over‐sized bone grafting. Truumees et al. tested the stress on various heights of graft in 18 fresh cadaver cervical spines ¹⁵ . Their data revealed that distractive and compressive forces were significant larger with 8 mm grafts than with 6 mm grafts. Within the range of passive physiological elongation of the ligament, a more powerful distractive force generates a more powerful compressive force. Excessively high grafting not only increases the difficulty of the surgery, but increases the incidence of graft collapse, end‐plate fracture and non‐union ¹⁴ , ¹⁵ . In the current study, implantation of a 160% baseline graft was more difficult and laceration of the annular ligament occurred in one specimen. In contrast, the surgical procedure for 140% baseline height grafting was much easier to perform without any complications.

Determination of the optimal height of strut‐grafting depends on multiple considerations besides the stability. An and his colleagues examined the height and area of the intervertebral foramen after C4–C5 discectomy and decompression in fresh cadaver spines ⁹ . They found that 2 to 3 mm distraction significantly increased the height and area, while more than 3 mm distraction did not result in enlargement of the foramen. However Bayley's group concluded that 5 mm was the optimal height of grafting in regard to the intervertebral foramen ¹⁷ . We also measured the height and area of the intervertebral foramen after C5–C6 discectomy; 140% baseline height grafting generated the best intervertebral foramen area in comparison with other sizes of grafts (unpublished data).

Conclusion

In summary, 140% baseline height strut‐grafting is best for maintaining the stability of the cervical spine following discectomy. This could be one important factor in helping clinicians to design their surgical strategy. In practice however, individual characteristics such as age and underlying diseases should not be ignored. The effects of graft height may also be affected by decreased visco‐elasticity, tissue calcification and the tolerance of the spinal cord.

Disclosure

The manuscript submitted does not contain information about medical device(s)/drug(s). No benefits in any form have been, or will be, received from a commercial party related directly or indirectly to the subject of this manuscript.