Ventral screw osteosynthesis is a common surgical method for treating fractures of the odontoid peg, but there is still no consensus about the number and diameter of the screws to be used. The purpose of this study was to develop a more accurate measurement technique for the morphometry of the odontoid peg (dens axis) and to provide a recommendation for ventral screw osteosynthesis.

Images of the cervical spine of 44 Caucasian patients, taken with a 64-line CT scanner, were evaluated using the measuring software MIMICS. All measurements were performed by two independent observers. Intraclass correlation coefficients were used to measure inter-rater variability.

The mean length of the odontoid peg was 39.76 mm (sd 2.68). The mean screw entry angle α was 59.45° (sd 3.45). The mean angle between the screw and the ventral border of C2 was 13.18° (sd 2.70), the maximum possible mean converging angle of two screws was 20.35° (sd 3.24). The measurements were obtained at the level of 66% of the total odontoid peg length and showed mean values of 8.36 mm (sd 0.84) for the inner diameter in the sagittal plane and 7.35 mm (sd 0.97) in the coronal plane. The mean outer diameter of the odontoid peg was 12.88 mm (sd 0.91) in the sagittal plane and 11.77 mm (sd 1.09) in the coronal plane. The results measured at the level of 90% of the total odontoid peg length were a mean of 6.12 mm (sd 1.14) for the sagittal inner diameter and 5.50 mm (sd 1.05) for the coronal inner diameter. The mean outer diameter of the odontoid peg was 11.10 mm (sd 1.0) in the sagittal plane and 10.00 mm (sd 1.07) in the coronal plane. In order to calculate the necessary screw length using 3.5 mm cannulated screws, 1.5 mm should be added to the measured odontoid peg length when anatomical reduction seems possible.

The cross-section of the odontoid peg is not circular but slightly elliptical, with a 10% greater diameter in the sagittal plane. In the majority of cases (70.5%) the odontoid peg offers enough room for two 3.5 mm cannulated cortical screws.

Cite this article: Bone Joint J 2013;95-B:536–42.

Introduction: Traumatic vertebral compression fractures (VCF) should be distinguished from fragility fractures, occurring as a result of decreased bone strength due to osteoporosis or cancer.

Polymethylmethacrylate cement (PMMA), as standard in fragility fractures, does not have the capacity to undergo remodeling. Therefore in young patients, a bio-compatible/–resorbable alternative would be preferable. KyphOs FS(R), a calcium magnesium hydroxyapatite cement has been developed for use during Balloon Kyphoplasty (BKP), a minimal invasive therapy.

This single-arm multicenter study evaluates the safety and effectiveness of this cement during BKP in young patients with stable VCFs.

We describe the clinical results up to 3mo of the ongoing 1-year follow-up study.

Methods: Male and female patients, aged 50 years or less, with up to 3 VCFs of type A1.1, A1.2 or A3.1, according to the Magerl/AO classification were included. The primary endpoint was the change from baseline in the 24 point Roland Morris Disability Questionnaire (RMDQ) score at seven days. Secondary endpoints included the quality of life as measured by EuroQol-5 Domain questionnaire (EQ-5D), the 10 point self-rated back pain (VAS) and device and/or procedure related adverse events.

Based on the standard deviation in the Wood study, up to 100 patients had to be enrolled to detect the minimal clinical important difference (MCID) of 2–3 pts on the primary endpoint.

Results: 50 patients out of 51 enrolled were eligible. The mean age was 36.4 years, 64% were male. 80% of the patients were treated for 1 VCF, 14% for 2 VCFs and 6% for 3 VCFs. At 3 mo, data of 45 patients were available. Mean RMDQ score at baseline was 20.29pts. The change from baseline in RMDQ at 7d was 9.42pts(95%CI 7.50–11.34, p< 0.0001) and 16.76pts(95%CI 15.21–18.30,p< 0.0001) at 3mo. Mean EQ-5D score at baseline was 0.16pts. The change from baseline in total EQ-5D at 7d was 0.52pts(95%CI 0.42–0.62,p< 0.0001) and 0.71pts(95%CI: 0.60–0.82,p< 0.0001) at 3mo. The change from baseline in VAS at 7d was 4.44pts(95%CI 3.80–5.08,p< 0.0001) and 5.43pts(95%CI 4.81–6.05,p< 0.0001) at 3mo. There were no device-related serious adverse events during the peri-operative period up to 3 mo.

Discussion: The MCID on the RMDQ is 2–3pts. In this study we obtained a difference of 9.42 points at 7 days. The recruitment was terminated earlier because of the highly significant results. The results on RMDQ were confirmed on all the other secondary endpoints with further improvement up to 3 mo.

Conclusion: The use of KyphOs FS(R) during BKP, appears to be a safe and effective method to treat traumatic VCFs in young patients. Longer follow-up is needed to confirm the results at 1 year.