Impact of constrained dual-screw anchorage on holding strength and the resistance to cyclic loading in anterior spinal deformity surgery: a comparative biomechanical study

Maintenance of spontaneous lumbar curve correction following thoracic fusion of main thoracic curves in adolescent idiopathic scoliosis

AIMS

The aims of our study were to provide long-term information on the behaviour of the thoracolumbar/lumbar (TL/L) curve after thoracic anterior correction and fusion (ASF) and to determine the impact of ASF on pulmonary function.

PATIENTS AND METHODS

A total of 41 patients (four males, 37 females) with main thoracic (MT) adolescent idiopathic scoliosis (AIS) treated with ASF were included. Mean age at surgery was 15.2 years (11 to 27). Mean follow-up period was 13.5 years (10 to 18).

RESULTS

For the TL/L curve, the mean curve flexibility evaluated with supine pre-operative bending radiographs was 78.6% (standard deviation 16.5%), with no significant loss of correction observed. On comparing patients with an increase of the TL/L curve increase (> 4º, n = 9, 22%) to those without, significant differences were observed in the correction rate of the MT curve at the final follow-up (p = 0.011), correction loss of the MT curve (p = 0.003) and the proportion of patients who had semi-rigid instrumentation (p = 0.003). Pre-operative percentage predicted forced vital capacity (%FVC) was 80%, dropping to 72% at final follow-up (p < 0.001). The Scoliosis Research Society questionnaire score was not significantly different between patients with and without a TL/L curve increase (p = 0.606). Spontaneous lumbar curve correction (SLCC) was maintained up to 18 years following selective ASF in most patients and demonstrated significant correlation with maintenance of MT curve correction.

CONCLUSION

Maintenance of MT curve correction using rigid instrumentation provided stable SLCC over time. An observed 8% decrease in %FVC indicates that ASF should be reserved for patients with no or only mild pulmonary impairment. Cite this article: Bone Joint J 2016;98-B:997-1002.

Biomechanical Comparison of Sacral Fixation Characteristics of Standard S1-Pedicle Screw Fixation versus a Novel Constrained S1-Dual-Screw Anchorage in the S1-Pedicle and S1-Alar Bone

STUDY DESIGN

Biomechanical Laboratory Study.

OBJECTIVE

Analysis of the biomechanical characteristics of a novel sacral constrained dual-screw fixation device (S1-PALA), combining a S1-pedicle screw and a S1-ala screw, compared to a standard bicortical S1-pedicle screw (S1-PS) fixation.

SUMMARY OF BACKGROUND DATA

Instrumented fusions to the sacrum are biomechanically challenging and plagued by a high risk of nonunion when S1-PS is used as the sole means of fixation. Thus, lumbopelvic fixation is increasingly selected instead, although associated with a reasonable number of instrumentation-related complications.

METHODS

Around 30 fresh-frozen human sacral bones were harvested and embedded after CT scans. Instrumentation was conducted in alternating order with bicortical 7.0 mm S1-PS and with the S1-PALA including a S1-PS screw and a S1-ala screw, of 7.0 and 6.0 mm diameter, respectively. Specimens were subjected to cyclic loading with increasing loads (25-250 N) until a maximum of 2000 cycles or displacement >2 mm occurred. All implant sacral units (ISUs) were subject to coaxial pullout tests. Failure load, number of ISUs surpassing 2000 cycles, number of cycles, and loads at failure were recorded and compared.

RESULTS

Donors' age averaged 77 ± 14.2 years, and BMD was 115 ± 64.8 mgCA-HA/ml. Total working length of screws implanted was 90 ± 8.6 mm in the S1-PALA group and 46 ± 5 mm in the S1-PS group (P = 0.0002). In the S1-PALA group, displacement >2 mm occurred after 845 ± 325 cycles at 149 ± 41 N compared to 512 ± 281 cycles at 106 ± 36 N in the S1-PS group (P = 0.004; P = 0.002). In coaxial pull-out testing, failure load was 2118.1 ± 1166 N at a displacement of 2.5 ± 1 mm in the S1-PALA group compared to 1375.6 ± 750.1 N at a displacement of 1.6 ± 0.5 mm in the S1-PS group (P = 0.0007; P = 0.0003).

CONCLUSION

The novel sacral constrained dual-screw anchorage (S1-PALA) significantly improved holding strength after cyclic loading compared to S1-PS. The S1-PALA demonstrated mechanical potential as a useful adjunct in the armamentarium of lumbosacral fixations indicated in cases that need advanced construct stability, but where instrumentation to the ilium or distal dissection to S2 should be avoided.

LEVEL OF EVIDENCE

N/A.

Pullout strength of thoracic pedicle screws improved with cortical bone ratio: a cadaveric study

BACKGROUND

The application of pedicle screw constructs for the osteoporotic vertebrae remains a serious clinical challenge for spinal surgeons and has been intensely studied recently. However, the exact role of the pedicular cortical bone composition and the screw-bone gap on the screw fixation failure has yet to be quantitatively documented. The current study aims to address this gap in our knowledge and elucidate possible relationships.

METHODS

Twelve fresh-frozen human cadaveric thoracic spine vertebrae (T9-T12) were harvested from six human cadavers (five males; one female; 63.5 ± 17 years). A three-dimensional reconstruction of the individual vertebrae was firstly rendered from computed tomography (CT) scan images to allow calculation of the cortical bone ratio. Specimens were then subdivided into three groups: Intact, 1-mm screw-bone gap, and 2-mm screw-bone gap. The gap groups were subjected to a standard cyclic fatigue-loading protocol. The pullout strength of the pedicle screws for all specimens were then determined.

RESULTS

The pullout strength of the 1-mm and 2-mm groups were significantly reduced when compared with the intact group. A moderate to excellent positive correlation was identified between the cortical bone area ratio and pullout strength for all groups (r > 0.55). A cortical shell ratio of 0.73 or higher was also found to be a safe cut-off index for screw fixation failure, even with an observable 1-mm screw-bone gap.

CONCLUSIONS

The current in vitro cadaveric spine study identified a significant correlation between cortical bone area ratio and the thoracic pedicle screw pullout strength. The presented results also demonstrate that the fatigue-loading-induced screw-bone gap of 1-mm was sufficient to cause a significant decrease in the pullout strength. However, a cortical bone area ratio of 0.73 or higher in this group was able to preserve most of the screw-bone interfacial strength, and subsequently may prevent a complete implant failure.