The best treatment method of large acetabular bone defects at revision THR remains controversial. Some of the factors that need consideration are the amount of residual pelvic bone removed during revision; the contact area between the residual pelvic bone and the new implant; and the influence of the new acetabular construct on the centre of rotation of the hip. The purpose of this study was to compare these variables in two of the most used surgical techniques used to reconstruct severe acetabular defects: the trabecular metal acetabular revision system (TMARS) and a custom triflanged acetabular component (CTAC).

Pre- and post-operative CT-scans were acquired from 11 patients who underwent revision THR with a TMARS construct for a Paprosky IIIB defect, 10 with pelvic discontinuity, at Royal Adelaide Hospital. The CT scans were used to generate computer models to virtually compare the TMARS and CTAC constructs using a semi-automated method. The TMARS construct model was calculated using postoperative CT scans while the CTAC constructs using the preoperative CT scans. The bone contact, centre of rotation, inclination, anteversion and reamed bone differences were calculated for both models.

There was a significant difference in the mean amount of bone reamed for the TMARS reconstructions (15,997 mm³) compared to the CTAC reconstructions (2292 mm³, p>0.01). There was no significant difference between overall implant bone contact (TMARS 5760mm² vs CTAC 5447mm², p=0.63). However, there was a significant difference for both cancellous (TMARS 4966mm² vs CTAC 2887mm², p=0.008) and cortical bone contact (TMARS 795mm² vs CTAC 2560mm², p=0.001). There was no difference in inclination and anteversion achieved. TMARS constructs resulted on average in a centre of rotations 7.4mm more lateral and 4.0mm more posterior.

Modelling of two different reconstructions of Paprosky IIIB defects demonstrated potential important differences between all variables investigated.

First-time revision acetabular components have a 36% re-revision rate at 10 years in Australia, with subsequent revisions known to have even worse results. Acetabular component migration >1mm at two years following revision THA is a surrogate for long term loosening. This study aimed to measure the migration of porous tantalum components used at revision surgery and investigate the effect of achieving press-fit and/or three-point fixation within acetabular bone.

Between May 2011 and March 2018, 55 patients (56 hips; 30 female, 25 male) underwent acetabular revision THR with a porous tantalum component, with a post-operative CT scan to assess implant to host bone contact achieved and Radiostereometric Analysis (RSA) examinations on day 2, 3 months, 1 and 2 years. A porous tantalum component was used because the defects treated (Paprosky IIa:IIb:IIc:IIIa:IIIb; 2:6:8:22:18; 13 with pelvic discontinuity) were either deemed too large or in a position preventing screw fixation of an implant with low coefficient of friction. Press-fit and three-point fixation of the implant was assessed intra-operatively and on postoperative imaging.

Three-point acetabular fixation was achieved in 51 hips (92%), 34 (62%) of which were press-fit. The mean implant to host bone contact achieved was 36% (range 9-71%). The majority (52/56, 93%) of components demonstrated acceptable early stability. Four components migrated >1mm proximally at two years (1.1, 3.2, 3.6 and 16.4mm). Three of these were in hips with Paprosky IIIB defects, including 2 with pelvic discontinuity. Neither press-fit nor three-point fixation was achieved for these three components and the cup to host bone contact achieved was low (30, 32 and 59%).

The majority of porous tantalum components had acceptable stability at two years following revision surgery despite treating large acetabular defects and poor bone quality. Components without press-fit or three-point fixation were associated with unacceptable amounts of early migration.

Roentgen Stereophotogrammetric Analysis (RSA) is the gold standard for measuring implant micromotion thereby predicting implant loosening. Early migration has been associated with the risk of long-term clinical failure. We used RSA to assess the stability of the Australian designed cementless hip stem (Paragon TM) and now report our 5-year results.

Fifty-three patients were prospectively and consecutively enrolled to receive a Paragon hip replacement. Tantalum beads were inserted into the bone as per RSA protocol and in the implant. RSA x-rays were taken at baseline 1–4 days post-surgery, at 6 weeks, 6 months, 12 months, 2 years, and 5 years. RSA was completed by an experienced, independent assessor.

We reported the 2-year results on 46 hips (ANZJS 91 (3) March 2021 p398) and now present the 5-year results on 27 hips. From the 2-year cohort 5 patients had died, 8 patients were uncontactable, 1 patient was too unwell to attend, 5 patients had relocated too far away and declined. At 5 years the mean axial subsidence of the stem was 0.66mm (0.05 to 2.96); the mean rotation into retroversion was 0.49˚ (−0.78˚ to 2.09˚), rotation of the stem into valgus was −0.23˚ (−0.627˚ to 1.56˚). There was no detectable increase in subsidence or rotation between 6 weeks and 5 years. We compared our data to that published for the Corail cementless stem and a similar pattern of migration was noted, however greater rotational stability was achieved with the Paragon stem over a comparable follow-up period.

The RSA results confirm that any minor motion of the Paragon cementless stem occurs in the first 6 weeks after which there is sustained stability for the next 5 years. The combination of a bi-planar wedge and transverse rectangular geometry provide excellent implant stability that is comparable to or better than other leading cementless stems.

First-time revision acetabular components have a 36% re-revision rate at 10 years in Australia, with subsequent revisions known to have even worse results. Acetabular component migration >1mm at two years following revision THA is a surrogate for long term loosening. This study aimed to measure the migration of porous tantalum components used at revision surgery and investigate the effect of achieving press-fit and/or three-point fixation within acetabular bone.

Between May 2011 and March 2018, 55 patients (56 hips; 30 female, 25 male) underwent acetabular revision THR with a porous tantalum component, with a post-operative CT scan to assess implant to host bone contact achieved and Radiostereometric Analysis (RSA) examinations on day 2, 3 months, 1 and 2 years. A porous tantalum component was used because the defects treated (Paprosky IIa:IIb:IIc:IIIa:IIIb; 2:6:8:22:18; 13 with pelvic discontinuity) were either deemed too large or in a position preventing screw fixation of an implant with low coefficient of friction. Press-fit and three-point fixation of the implant was assessed intra-operatively and on postoperative imaging.

Three-point acetabular fixation was achieved in 51 hips (92%), 34 (62%) of which were press-fit. The mean implant to host bone contact achieved was 36% (range 9–71%). The majority (52/56, 93%) of components demonstrated acceptable early stability. Four components migrated >1mm proximally at two years (1.1, 3.2, 3.6 and 16.4mm). Three of these were in hips with Paprosky IIIB defects, including 2 with pelvic discontinuity. Neither press-fit nor three-point fixation was achieved for these three components and the cup to host bone contact achieved was low (30, 32 and 59%).

The majority of porous tantalum components had acceptable stability at two years following revision surgery despite treating large acetabular defects and poor bone quality. Components without press-fit or three-point fixation were associated with unacceptable amounts of early migration.

Femoral impaction bone grafting (IBG) may be used to restore bone stock in revision total hip arthroplasty (THA) and allow use of a shorter, than otherwise, length prosthesis. This is most beneficial in young patients who are more likely to require further revision surgery. This study aimed to assess the results of femoral IBG for staged revision THA for infection. A prospective cohort of 29 patients who underwent staged revision THA for infection with femoral IBG and a cemented polished double-tapered (CPDT) stem at the final reconstruction was investigated.

The minimum follow-up was two years (2 – 10 years, median 6 years). Stem subsidence was measured with radiostereometric analysis. Clinical outcomes were assessed with the Harris Hip, Harris Pain, and and Société Internationale de Chirurgie Orthopédique et de Traumatologie Activity (SICOT) Scores. The original infection was eradicated in 28 patients. One patient required a repeat staged revision due to re-infection with the same organism. At two-year follow-up, the median subsidence at the stem-bone interface was −1.70 mm (−0.31 to −4.98mm). The median Harris Hip Score improved from 51 pre-operatively to 80 at two years (p=0.000), the Harris Pain Score from 20 to 44 (p=0.000) and the SICOT Score from 2.5 to 3 (p=0.003).

As successful eradication of infection was achieved in the majority of patients and the stem migration was similar to that of a primary CPDT stem, this study supports the use of femoral IBG during the final reconstruction of the femur after staged revision THA for infection.

Aims

The purpose of this exploratory study was to investigate if the 24-hour activity profile (i.e. waking activities and sleep) objectively measured using wrist-worn accelerometry of patients scheduled for total hip arthroplasty (THA) improves postoperatively.

Introduction & aims

The magnitude and pattern of acceptable migration in clinically successful cementless stems is not well understood. Radiostereometric analysis (RSA) is a well-recognised method of assessing implant migration.

Previous studies have reported long term RSA results for cemented stems. The aim of this study was to assess the long-term migration characteristics of the Corail hydroxyapatite-coated cementless stem at 10 years using RSA.

Introduction

Severely comminuted, displaced acetabular fractures with articular impaction in the elderly population present significant treatment challenges. To allow early post-operative rehabilitation and limit the sequelae of immobility, treatment with acute total hip replacement (THA) has been advocated in selected patients. Achieving primary stability of the acetabular cup without early migration is challenging and there is no current consensus on the optimum method of acetabular reconstruction. We present clinical results and radiostereometric analysis of trabecular metal (TM) cup cage construct reconstruction in immediate THA without acetabular fracture fixation.

Acetabular components used to treat large defects are at greater risk of loosening. Porous tantalum acetabular components have reported the most promising early to midterm revision rates. Early stability of acetabular components used at revision THR was shown to be a good predictor of later loosening. The primary aim was to assess the migration of porous acetabular component used to reconstruct severe acetabular defects. Secondarily, we investigated the effect of acetabular defect severity and type of component fixation on migration.

Radiosterometric analysis was used to measure migration at a mean follow-up of four years, (range 2–10) in 59 reconstructions of severe acetabular defects with porous tantalum components. Acetabular component fixation was classified as superior if augmented with screws through cup, augments or cage in the ilium only. Fixation was classified as combined, superior and inferior, if flanges and/or screws were also placed in the ischium and or pubis. Acceptable limits of proximal migration were defined as ≤1mm within 2 years and ≤2.5mm at any time point.

Eight hips had reconstruction of Paprosky II defects with superior fixation only. The mean proximal migration of the eight acetabular components was 0.25mm (0.08–0.40) at 2 years and 0.29mm (0.10–0.81) at last follow-up.

Fifty-one hips had reconstruction of Paprosky III defects. Seven of these reconstructions exceeded the migration thresholds. Five reconstructions (four with superior fixation and one cup cage construct with no inferior screw fixation) of hips with pelvic discontinuity developed pain and were re-revised for loosening. Two reconstructions are asymptomatic and migrated 2.68mm (cup-cage construct with superior screws) and 2.86mm (no pelvic discontinuity, superior fixation) at final follow-up. The mean proximal migration of the 51 Paprosky III reconstructions was 0.99mm (0.03 to 16.4) at 2 years and 1.92mm (0.01 to 29.4) at last follow-up. The mean proximal translation at 2 years of the 11 reconstructions with inferior screw fixation was 0.2mm (−0.6 to 0.7mm), compared with 0.9mm (−0.6 to 16.4mm) for the reconstructions without inferior screw fixation.

In conclusion, when used to reconstruct Paprosky II defects, porous tantalum acetabular components provide component stability similar to a good performing primary THR. These implants achieve adequate stability when used to treat Paprosky III defects, including those with pelvic discontinuity. For the most severe defects, combined fixation with inferior screws is recommended, particularly when reconstructing hips with pelvic discontinuity.

Summary

Application of RSA in supine and standing positions allows pelvic fracture stability to be measured more accurately than current techniques. RSA may enable a better understanding of these injuries.

Introduction

To evaluate the clinical success and hip pain and function of patients with infected hip replacement treated by two-stage exchange using a temporary implant with high dose vancomycin added to the antibiotic cement at the first stage revision.

Introduction

There has been almost universal adoption of highly cross-linked polyethylene as the polyethylene of choice in metal-on-polyethylene articulations in total hip replacement (THR). Although wear of conventional polyethylene has been shown to be related to periprosthetic osteolysis, the relationship between wear of highly cross-linked polyethylene and osteolysis remains uncertain. Our aim was to determine the incidence and volume of periacetabular osteolysis at a minimum of seven years following primary THR with metal on highly cross-linked polyethylene articulations.

Anterior lumbar inter-body fusion (ALIF) is a surgical procedure that is available to chronic lower back pain patients who fail to respond to conservative treatments. Failure to achieve fusion may result in persistence of pain. Fusion of the lumber vertebral segment is more accurately assessed using fine-cut helical Computed tomography (CT) scans (0.25 mm thickness slices). Unfortunately this technique exposes the body to high radiation dose with hazard of increase risk of late malignancy. An alternative imaging tool is radiostereometry (RSA) which developed as a means to determine the magnitude of relative motion between two rigid bodies. In this study we used RSA to detect movement at the fused lumbar segment (ALIF site) during flexion and extension and compare the results obtained with fine-cut helical CT scan using histopathology as final gold standard assessment tool.

ALIF of three levels of lumbar spine (L1-L2, L3-L4, and L5-L6) was done in 9 sheep. The sheep divided into three groups (3sheep each). The first group had RSA assessment immediately, 3, and 6 months after surgery. The second group had RSA immediately, 3, 6, 9 months after surgery. The third group had an RSA immediately, 3, 6, 9, 12 months after surgery All the animals were humanly killed immediately after having the last scheduled RSA (group1, group2, and group 3 sheep were killed 6 month, 9month and 12 months after surgery respectively). This followed by in vitro fine cut CT and histopathology after the animals are scarified. Micro CT scan has been also used to identify the area where histopathology slide should be made to pick up fusion. Fine cut CT scan assessment for all sheep were done. The CT scan has been reported by two independent radiologists. Histopathology has been started and will finish in 2 weeks

RSA showed there was significant increasing stiffness of the spine though the fused segments as the time pass on compare to immediate postoperative assessment. CT scan were done and showed variable fusion though out the spinal segments. Histopathology of all sheep has been started and the results will be available in 2 weeks which will be followed by statistical assessment to decide how accurate RSA compare to CT scan in assessment of fusion.

This study aimed to compare the early clinical results and stem subsidence between three consecutive series of revision hip replacement cases with femoral impaction bone grafting to evaluate the effects of developments in technique. In the original series 1 (n=23), bone graft was irradiated at 25kG. I n series 2 (n=12) non-irradiated double washed graft and long stems were used as required.

In series 3 (n=21) modular tamps were used. Sensitive radiographic analysis techniques, EBRA and RSA, were used to measure stem subsidence. Major stem re-revision was required in five hips in series one, one hip in series two and no hips in series three. Two periprosthetic fractures occurred in series one. There was a statistically significant reduction in stem subsidence at the cement-bone interface at 12 months between series one and series two and three (p<0.05). In series three there was negligible stem subsidence at the cement-bone interface.

Technique developments in femoral impaction grafting, including the use of modular tamps designed to simply the procedure, yields excellent early clinical and radiographic results. Using RSA, we have shown that the fixation of the stems in bone is comparable to that achieved in primary hip replacement.

Introduction: In the past, surgeons have found impaction bone grafting technically difficult leading to its limited use. This paper reviews the long term results and developments in instrumentation and techniques aimed at simplifying femoral impaction grafting at revision hip replacement. The expanded indications for this procedure are reviewed and recent results of stem fixation using radiostereometric analysis (RSA) are reported.

Methods: The impaction bone grafting procedure, using a cemented collarless polished double taper stem, has been used in 66 hips (median patient age 63yrs) since 1993. The technique has undergone numerous developments. Modular tamps have been used in the last 29 hips and in the last seven hips, a pneumatic mechanical vibration device has been used in place of manual impaction. Stem subsidence at both the prosthesis-cement and cement-graft/bone interfaces was measured, more recently using radiostereometric analysis.

Results: There was a high early incidence of failure in the initial cases but there have been no further revisions for mechanical failure at up to 15 years. Technique developments have resulted in dramatic improvements in stem fixation achieved. In the modular tamp cases, the mean stem subsidence at the cement-bone interface at 12 months was 0.07mm (0 to 0.7mm) at 12 months. The stem subsidence in the hips where the mechanical vibration device was used was 0.05mm (0 to 0.06mm). Femoral impaction grafting offers special advantages in younger patients include standard femoral stem revision and at the second stage of two stage revision for infection.

Discussion and Conclusion: A stable cement-bone interface is achieved using advanced techniques of femoral impaction grafting that includes the use of modular impaction instruments. Early results of mechanical vibration impaction are encouraging. Femoral impaction grafting restores bone and new techniques simplify the femoral revision procedure.

Introduction: Differentially loaded radiostereometric analysis (DLRSA) uses RSA whilst simultaneously applying load to the bones under investigation. This technique allows measurement of interfragmentary displacements under measured weightbearing and joint movement. We have used this technique to prospectively monitor tibial plateau fractures and present the results of the first nine patients with six month follow up.

Method: Nine 41-B3 fractures were treated with open reduction internal fixation by one surgeon. At operation, RSA beads were inserted in the depressed osteochondral fragment and the adjacent non-fractured metaphysis. Postoperative weightbearing was restricted to 20kg and knee flexion to 60° for the first six weeks. Follow up included clinical and radiological examinations and patient reported outcome scores (Lysholm, KOOS). DLRSA examinations included RSA radiographs in 60° flexion and under measured weightbearing at six weekly intervals up to six months postoperatively. Significant interfragmentary displacement was defined as translations greater than 0.5mm and/or rotations greater than 1.5°.

Results: No postoperative displacement was identified on plain radiographs, except in one patient who fell two weeks postoperatively.

RSA: Longitudinal Results: In all patients, the osteo-chondral fragment continued to migrate up to six months, with one exception that stabilised at three months. At six months, the osteochondral fragment translated between 0.02 and 4.15 mm and rotated between 0.2 and 7.2° (> 0.5mm and/or > 1.5° in five cases).

Discussion: Tibial plateau fractures continue to migrate up to six months after treatment. Active range of motion, partial weightbearing to six weeks and weightbearing up to one body weight after six weeks was proven a safe postoperative regimen. Greater displacements recorded over time may be attributed to loading of more than one body weight, for example, the patient that fell recorded the largest amount of migration over time.

Differentially loaded radiostereometric analysis (DLRSA) uses RSA whilst simultaneously applying load to the bones under investigation. This technique allows measurement of interfragmentary translations and rotations under measured weight bearing and joint movement. We have recently introduced this technique to monitor tibial plateau fracture healing. This paper presents our preliminary results.

Twelve patients with a 41 B2, B3, C2, or C3 fracture were followed for a minimum of three months. RSA beads were inserted in the largest osteochondral fragment and the adjacent metaphysis. Knee flexion was restricted to 60° for 6 weeks. After partial weight bearing (20kg) between 2 and 6 weeks, patients progressed to full weight bearing. Follow up included clinical and radiological examinations and patient reported outcome scores (Lysholm, KOOS). DLRSA examinations included RSA radiographs in 60° flexion and under measured weight bearing. Significant interfragmentary displacement was defined as translations greater than 0.5mm and/or rotations greater than 1.5°.

There was no loss to follow-up. Longitudinal RSA follow-up: Follow-up RSA radiographs were compared to postoperative examinations. Osteochondral fragment depression was less than 0.5mm in seven patients and between 2 and 4mm in the remaining five patients. Significant interfragmentary displacement after three months was recorded in three patients. DLRSA flexion results: Under 60° of flexion, translations over 0.5mm were recorded in five patients (one postoperatively; one at 2 weeks; two at 6 weeks; and one postoperatively, at 2 weeks and at 3 months). Rotations over 1.5° were recorded in six patients (one postoperatively; two at 2 weeks; one at 6 weeks; one at 2 weeks, 3 months and 4.5 months; and one postoperatively, at 2 weeks, 3 months and 6 months). DLRSA weight bearing results: Under partial weight bearing at two weeks, two patients recorded significant translations, one involving a significant rotation. Under weight bearing as tolerated, three patients recorded significant translations (one at 6 weeks; and two at 18 weeks) and four patients recorded significant rotations (one at 6 weeks; one at 18 weeks; and two at 12 and 18 weeks). Patient Reported Outcomes: Both the Lysholm and KOOS scores improved between 6 weeks and 3 months. DLRSA provides new insight and perspective in tibial plateau fractures. Some fractures take more than three months to heal. Our current rehabilitation protocol was safe in most patients, however significant interfragmentary displacement was encountered in 17% at the 2 week followup, raising questions about the quality of the initial stability.

Differentially loaded radiostereometric analysis (DLRSA) uses RSA whilst simultaneously applying load to the bones under investigation. This technique allows measurement of interfragmentary translations and rotations under measured amounts of weight bearing. The aim of this paper was to measure the mechanical stiffness of distal femoral fractures during healing.

Six patients with a 33A2, 33A3, 33B2 and 33C2 fracture were treated with open reduction, internal fixation using a long bridging plate. All patients had a DLRSA examination at 6, 12, 18 and 26 weeks postoperatively. Each DLRSA examination consisted of RSA radiographs taken without load (pre-load), under different increments of load, and finally, without load (post-load). The direction and magnitude of the interfragmentary displacements in six degrees of freedom were recorded at each examination.

DLRSA examinations were able to monitor the inter-fragmentary displacements of the distal femoral fragment relative to the femoral shaft. The interfragmentary displacement recorded, progressively increased as more load was applied in all patients, at all follow-up time points. The two dimensional (2D) translations under maximum tolerated load, progressively decreased over time in three patients. The 2D translations recorded under 60 kg of load at 26 weeks for these patients was 0.18, 0.21 and 0.27mm. The 2D translations of two patients did not decrease progressively between 6 and 18 weeks but did decrease at 26 weeks to 0.47 and 0.75mm. One patient recorded 2D translations of 4.11, 3.48 and 4.53mm under 30kg at 12, 18 and 26 weeks respectively. In the majority of examinations, post-load radiographs enabled the interfragmentary displacements under load to be identified as elastic in nature.

The DLRSA stiffness data confirmed that at 26 weeks three patients had united; two were delayed but improving; and one was a clear non-union without progression. DLRSA examinations may be used as a clinical research tool. to monitor in vivo the stiffness of healing femoral fractures fixed with “relative stability”.

Introduction: Interspinous implants have recently been proposed as an alternative in the treatment of lumbar spinal stenosis and foraminal stenosis. Interspinous implants are thought to unload the facet joints, restore foraminal height and provide improved spinal stability especially in extension with a minimally invasive approach. It has been proposed that, combined with a tension band, stabilisation could also be obtained in flexion, thus avoiding the need for pedicle screw fixation. Little biomechanical data exists to support these notions. The aim of this in vivo study was to investigate the effect of a novel, minimally invasive, unilaterally inserted interspinous implant on flexion-extension range of motion of the lumbar spine.

Methods: Following the induction of general anesthesia, ten adolescent Merino lambs (24–30 kg) underwent a destabilisation procedure at the level of L1–L2, thus simulating a stenotic degenerative spondylolisthesis, as described previously. All animals were placed in a sidelying posture and lateral radiographs were taken in the neutral posture and in end stage flexion and extension trunk positions with the central ray at the level of L1–L2. The flexion manouvre was reproduced in each subject by securing a rope above the carpus (forelimb) and the tarsus (hindlimb). This same radiographic protocol was repeated following the insertion of an 8 mm InSwing interspinous device at L1–L2, and again with the implant secured by means of a tension band tightened to 1 N/m around the L1 and L2 spinous processes. Care was given to respect the integrity of the supra-spinous ligament. The insertion technique requires a minimally invasive unilateral approach, therefore leaving the attachment of the erector spinae muscle on the contralateral side intact. Using Cobb’s method, intersegmental range of motion (ROM) was assessed in each of the conditions and compared. A paired t-test compared ROM for each of the experimental conditions (P< .05).

Results: The addition of the InSwing interspinous implant reduced mean total flexion-extension ROM from 6.3 degrees to 5.3 degrees which was further reduced to 3.6 degrees with the device secured by means of a tension band. These differences were not statistically significant, but the addition of the tension band to the interspinous device resulted in a significant reduction of lumbar flexion (p< .05).

Discussion: The interspinous device tended to reduce the total flexion-extension ROM at the level of the implant, however the results were not significant. The addition of a tension band was found to significantly stabilize the spine in flexion. To our knowledge, this is the first in vivo study radiographically showing the advantage of using an interspinous device, specifically InSwing, to stabilize the spine in flexion. These results are important findings particularly for patients with clinical symptoms of instable degenerative spondylolisthesis.

Introduction: Spondylolysis in skeletally immature individuals is common but only a small proportion of cases develop pain and spondylolisthesis. The pattern of instability and pathologic consequences of surgically created spondylolytic defects have not previously been described. An animal model of lumbar spondylolysis was created using skeletally immature sheep with the aim of observing the intervertebral mobility and pathologic consequences of creating a spondylolytic segment.

Methods: Bilateral spondylolytic defects were created in the fifth lumbar vertebrae of fourteen sheep aged approximately 16 weeks using a posterior surgical approach. Using tantalum markers that were placed in the lumbar spine segments in the vicinity of the lesion, three dimensional translations and rotations in flexion and extension were recorded under general anaesthetic using Radiostereometric Analysis (RSA) at the time of surgery. A novel method of ovine spine manipulation was developed. Briefly, with the animal in a lateral decubitus position spinal extension was achieved by fixing the shoulders and hips in full extension. Similarly, the limbs were brought into apposition to achieve spinal flexion. A control group of seven sheep had tantalum markers only in the same lumbar spine segments. The animals were re-tested under general anaesthetic at six weeks and six months following surgery. After six months the animals were sacrificed and their spines examined using CT and microscopic analysis.

Results: Bilateral spondylolytic defects did not result in significant transverse (x-axis) translation. RSA showed significant differences between the spondylolytic group (mean 1.22°, range 0.30–3.74) and controls (mean 0.28°, range 0.03–0.77)) for rotation in the axial plane (y-axis, p< 0.0025) immediately after surgery. The spines radiologically stabilised over six months.

Discussion: Surgically induced bilateral spondylolytic defects in this immature ovine model result in increased rotation in the axial plane but do not create transverse translation (spondylolisthesis).