Introduction: It has long been recognised that static plain x-rays are a sub-optimal method for the assessment of lumbar fusion. Blumenthal and Gil showed that radiographic assessment of fusion corresponded with operative findings only 69% of the time. Santos et al suggest that both plain x-rays and flexion/extension x-rays overestimate the fusion rate when compared to helical computed tomography (CT). To date there has been no correlation of CT assessment of fusion with surgical exploration. In this study we present an animal model of lumbar spine pseudarthrosis and compare three imaging modalities with micro-cut CT scanning and cadaveric assessment.

Methods: Approval was gained from the QUT animal ethics committee. Eleven mixed bred ewes were assigned to either a fusion group or an intentional pseudarthrosis (IP) group. A dorsal approach to the facet joints of L2/3 was made. The facet joints were destabilised by resecting the articulating surfaces with a rongeur. In the fusion group, the spinous processes of the destabilised segment were wired tightly together and a bone graft harvested from the iliac crest was placed into the joint space. In the IP group the bone graft bed was prepared similarly except that a small proportion of the articulating surface was left intact and a 1.0 cm2 roll of oxidised cellulose was placed into the facet joint space bilaterally. In the IP group the spinous processes were wired around an interspinous spacer which was later removed to create a similar degree of laxity in the fixation of each of the IP specimens. The animals were sacrificed at 6 months and static and dynamic lateral radiographs obtained. The spine was removed en bloc, and high speed fine cut (2mm) CT Scanning performed. The specimens were individually assessed for fusion by micro-cut CT scanning. Eight independent, blinded orthopaedic surgeons, were asked whether they considered the spine to be fused based on

plain x-ray

These results were correlated with a fusion rate based on the micro CT. The specificity and sensitivity of these radiological measures in diagnosing pseudarthrosis and inter-rater reliability using Fleiss’ Kappa scores for each method were calculated.

Results: For assessing pseudarthrosis identified by microCT the plain film sensitivity was 0.41 and the specificity was 0.47. For assessing pseudarthrosis with plain and flexion extension xrays the sensitivity was 0.55 and the specificity was 0.33. For assessing pseudarthrosis with plain flexion extension xrays and CT the sensitivity was 0.81 and the specificity was 0.88. The Kappa score for plain films was 0.15, for flexion extension was 0.07 and CT was 0.54.

Discussion: This study suggests that plain radiographs and flexion extension radiographs are an unreliable measure of posterior lumbar fusion. The current clinical gold standard for assessment of fusion (CT) was able to correctly identify non-union in 80% of cases. Whilst no alternatives to structural assessment of the fusion mass with CT currently exist it is important to recognise the limitations of this technique.

In recent years, plate osteosynthesis in metaphyseal and diaphyseal long bone fractures has been increasingly applied in a minimally invasive fashion. Several clinical studies describe a beneficial effect of the smaller additional soft tissue damage, resulting in satisfying fracture and soft tissue healing. However, is the surgical soft tissue damage really evidently smaller and the recovery faster?

A trauma model with severe, circumferential soft tissue damage to the distal right thigh and a distal multifragmentary (AO type C) femur fracture was carried out on 24 male sheep. After five days temporary external fixation, an internal fixator was placed either by a conventional open lateral approach or by minimally invasive technique. To assess the soft tissue damage and its recovery within the first 14 days, local compartment pressure monitoring as well as daily measurements of systemic markers (Creatin Kinase, CK and Lactate Dehydrogenase, LDH) in blood were performed. The local monitoring with a special probe (Neurovent PTO, Raumedic AG, Germany) within the quadriceps muscle allowed the measurement of compartment pressure (CP), as well as temperature.

The CK and LDH levels responded to the severe trauma with high peaks within the first 48 hours post trauma. After the internal fixator operations CK levels illustrate a significantly lower increase (p< 0.05) in the minimally invasive group compared to the open approach group in the first two days postoperatively. LDH levels show lower values for the minimally invasive group (p=0.06).

The values of CP present an initial increase after the trauma and then higher values (p=0.08) after the open plating operation. For the intracompartmental temperature no statistical differences were found, too (p=0.17).

These results, with reduced additional soft tissue damage and faster recovery in the minimally invasive approach group, reflect the clinical experience and expectations. However, while minimally invasive plate osteo-synthesis is certainly a desired option for fracture fixation, good surgical skills are required to insure that the reduced surgical trauma is in line with optimal fracture healing. The influence of the two different approaches on the bone healing per se, as well as the influence on soft tissue functionality, has yet to be demonstrated.

Introduction Cement leakage into adjacent structures is the main complication during vertebroplasty. The majority of these leaks are asymptomatic, but pulmonary cement embolism has been reported to cause cardiovascular disturbances and even death (1,2). Furthermore, the use of calcium phosphate (CaP) cements for vertebroplasty may aggravate cardiovascular deterioration in the event of cement embolism by stimulating coagulation [3].

The cardiovascular effects of pulmonary cement embolism were investigated using an animal model.

Methods In 18 skeletally mature sheep, 2.0ml cement was injected into the pulmonary trunk during general anaesthesia (approved by Animal Ethics Committee). Three different cements were used: 1) PMMA (Simplex P, Stryker); 2) PMMA with 10% hydroxyapatite (PMMA & HA) (Vertecem, Synthes); 3) Experimental injectable CaP cement (Synthes). The following cardiovascular parameters were recorded continuously (endpoint: 60min post-injection): arterial, central venous, pulmonary arterial pressures and cardiac output. Blood gases and coagulation parameters (antithrombin, D-dimer, prothrombin fragments I & II) were measured pre-injection, 10, 30 and 60min post-injection. Postmortem, lungs were removed intact and submitted to computer tomography (CT) imaging.

Results There were no fatalities. After 1min, mean pulmonary arterial pressure had increased by 9%, 14% and 21% from pre-injection value in the PMMA, PMMA & HA and CaP group respectively. Differences in pulmonary arterial pressure between the three material groups were not statistically significant. Pulmonary arterial pressure stayed elevated for the duration of the experiment (i.e. 60min post-injection). There were no other significant changes in cardiovascular, blood gas or coagulation parameters from pre- to post-injection values. Three dimensional reconstructions of the CT images showed a tendency of the CaP cement to break up into multiple smaller pieces whereas the two other cements did not.

Discussion Cement embolism led to mild pulmonary hypertension in all material groups. Present results are in contrast to earlier reports (pig model) of fulminant cardiovascular deterioration after CaP cement embolism (3). Present changes were not as severe and there was no evidence of thromboembolism. This discrepancy may have been due to differences in the cement formulations or the animal model.

Pulmonary hypertension was more severe in the CaP cement group. This may have been due to the disintegration of the CaP cement resulting in blockage of more pulmonary vessels compared to the PMMA cements.