Introduction: Thickened ligamentum flavum (LF) is a major contributor to the clinical syndrome of lumbar canal stenosis (LCS). The patho-mechanisms responsible for this phenomenon remain unclear. Cysts adjacent to facet joints (FJ) in the spine are regarded as rare entities that may uncommonly contribute to LCS. Inaccurate pathological interpretation and unawareness of a key anatomical feature has generated erratic terminology and confusion about their origin.

Methods: Twenty-seven consecutive patients with radiologically confirmed central canal or lateral recess stenosis underwent lumbar laminectomy for neurogenic symptoms. Surgical specimens comprising en bloc excision of LF and medial inferior facet (to retain LF and FJ relationships) were examined microscopically following staining with haematoxylin-eosin and Miller’s elastic stain. Controls were facet/LF specimens from 89 cadaver lumbar spines.

Results: Mean LF thickness was 8.9 mm (+/− 0.3 mm SEM) at the operated levels and 2.9 mm (+/− 0.3 mm) at the non-operated, adjacent levels (p < 0.01). Twenty-eight synovial cysts (8 bilateral, 12 unilateral) were present at a single level in 20 (74%) patients. Synovial cysts per spine level were: L1/2 = 0; L2/3 = 3; L3/4 = 7; L4/5 = 16; L5/S1 = 2. The cyst levels all showed advanced osteoarthritis and LF degeneration. Ten patients (50 %) with cysts had pre-existing degenerative spondylolisthesis (DS). Only 5 patients had pre-operative radiological apperances of unilateral facet cysts. Therefore 82 % of our observed synovial cysts were microscopic or occult. The synovial cysts communicated with the FJ via a bursa-like cleft within the LF, and their linings of synoviocytes and other cells contained fragments shed from the articular surface. The control cadaver specimens revealed that a synovial bursa or intra-ligamentous out-pouching from the synovial cavity was present in 90% of normal LF at L4/5 and was up to 12 mm in length. This intra-ligamentous synovial recess, either wholly or partially lined by synoviocytes, was only present in 55% of specimens at L1/2 with a maximum length of 5 mm. Several other juxtafacet cyst types were observed in the experimental group and a novel classification based upon pathological findings is presented.

Discussion: Para-facetal intraspinal cysts are common in degenerative lumbar spinal stenosis. DS is also a frequent finding but is statistically unrelated to cyst formation (Chi-square: p=0.187). We have found that debris from osteoarthritic facet joints enters a bursa-like cleft within the LF where it becomes incorporated into the wall where it excites a granulomatous reaction leading to blockage and synovial cyst formation. The existence of this channel has not been reported previously. We suggest that microscopic synovial cysts contribute significantly to the ligamentous thickening seen in LCS. We also present a novel classification of juxtafacet cysts based on our pathological findings.

Introduction: The age-related reduction of water-trapping proteoglycans needed to maintain optimal disc hydration may be caused by reduced synthetic ability or fewer chondrocytes but there is a paucity of objective quantitative studies of disc cellularity.

Methods: Sagittal sections of L4–5 discs were subdivided into a nucleus zone and 18 annulus zones prior to determining cell density (cells/mm2) in

the mid-sagittal plane of 10 male and 10 female discs aged 13–79 years;

Results:

Most chondrocytes were unicellular but bicellular and multicellular chondrons were common in the margins of large tears and the nucleus in degenerate discs.

Discussion: The findings that cell density is higher in the posterior annulus and in the right half of the disc, tends to be increased if the cartilage end-plate is thicker, and is not uniformly diminished by large tears, indicates that disc cellularity is influenced by a complex interplay of factors which needs to be understood before attempts are made to restore the structural and functional integrity of degenerate discs.

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).

Introduction: Intradiscal electrothermal therapy (IDET) is being used increasingly as a minimally-invasive treatment for chronic discogenic low back pain, with success reported in up to 70% of cases. The mechanism of action however is poorly understood. Proposed mechanisms include the contraction of collagen and the coagulation of annular nociceptors. An ovine model was used to assess the innervation of peripheral posterolateral annular lesions and the potential for IDET to denervate this region.

Methods: Posterolateral annular incisions were made in 36 lumbar discs of 18 sheep. After twelve weeks the sheep underwent IDET at one level and a sham treatment at the other level. IDET was performed using a modified intradiscal catheter (SpineCATH™, Oratec Interventions Inc., Menlo Park, CA). Temperatures were recorded in the nucleus and the posterior annulus. The spines were harvested at intervals of up to eighteen months. Histological sections of the discs were stained with haematoxylin and eosin and an antibody to the general neuronal marker PGP 9.5.

Results: The target temperature of 90°C at the catheter tip was reached in all cases. The mean maximum T_Pa was 63.6°C and the mean maximum TN was 67.8°C. Vascular granulation tissue consistent with a healing response was observed in the region of the posterior annulus tear of all incised discs from 12 weeks. PGP 9.5 positive nerve fibres were clearly identified in the adjacent periannular tissue, but were scarce within the outer few lamellae of the annulus. There were no fewer nerve fibres identified in those specimens that had undergone IDET. From six weeks after IDET there was evidence of thermal necrosis in the inner annulus, sparing the periphery of the disc.

Discussion: IDET delivered at 90°C in the sheep consistently heats the posterior annulus and the nucleus to a temperature associated with coagulation of nociceptors and collagen contraction. Thermal necrosis was observed within the inner annulus from six weeks after IDET. In this model IDET did not appear to produce denervation of the posterior annular lesion.

INTRODUCTION: Intradiscal electrothermal therapy (IDET) is being used increasingly as a minimally-invasive treatment for chronic discogenic low back pain, with success reported in up to 70% of cases. The mechanism of action however is poorly understood. Proposed mechanisms include the contraction of collagen and the coagulation of annular nociceptors. An ovine model was used to assess the innervation of peripheral posterolateral annular lesions and the potential for IDET to denervate this region.

METHODS: Posterolateral annular incisions were made in 36 lumbar discs of 18 sheep. After twelve weeks the sheep underwent IDET at one level and a sham treatment at the other level. IDET was performed using a modified intradiscal catheter (SpineCATH^TM, Oratec Interventions Inc., Menlo Park, CA). Temperatures were recorded in the nucleus and the posterior annulus. The spines were harvested at intervals of up to eighteen months. Histological sections of the discs were stained with haematoxylin and eosin and an antibody to the general neuronal marker PGP 9.5.

RESULTS: The target temperature of 90°C at the catheter tip was reached in all cases. The mean maximum T_Pa was 63.6°C and the mean maximum T_N was 67.8°C. Vascular granulation tissue consistent with a healing response was observed in the region of the posterior annulus tear of all incised discs from 12 weeks. PGP 9.5 positive nerve fibres were clearly identified in the adjacent periannular tissue, but were scarce within the outer few lamellae of the annulus. There were no fewer nerve fibres identified in those specimens that had undergone IDET. From six weeks after IDET there was evidence of thermal necrosis in the inner annulus, sparing the periphery of the disc.

DISCUSSION: IDET delivered at 90°C in the sheep consistently heats the posterior annulus and the nucleus to a temperature associated with coagulation of nociceptors and collagen contraction. Thermal necrosis was observed within the inner annulus from six weeks after IDET. In this model IDET did not appear to produce denervation of the posterior annular lesion.

To assess the potential for IDET to ablate nerve fibres in an experimentally induced peripheral annular lesion.

Intradiscal electrothermal therapy (IDET) is being increasingly used as a minimally-invasive treatment for discogenic low back pain, with success reported in up to 70% of cases. One proposed mechanism of IDET is ablation reported in up to 70% of cases. One proposed mechanism of IDET is ablation of nerve fibres in the peripheral annulus. An ovine model was used to assess the innervation of peripheral annular lesions and the potential for IDET to denervate this region of the disc.

Postero-lateral annular incisions were made in 32 lumbar discs of 16 sheep. At twelve weeks the sheep underwent IDET at one level and a sham treatment at the other level. IDET was performed using a modified Intradiscal Catheter (SpineCath, Oratec Interventions Inc., Menlo Park, CA). The spines were harvested at intervals up to six months. Histological sections of the discs were stained with H& E and an antibody to the general neuronal marker PGP 9.5.

Vascular granulation tissue consistent with a healing posterior annular tear was observed in all incised discs from 12 weeks, extending to an average depth of 850 μm at 0 weeks to 690 μm at 6 months. PGP 9.5 positive nerve fibres were clearly identified outside the discs but were scarce within the discs. Nerves were identified up to 300 μm inside the annulus, from the earliest time point, and there was a trend towards less innervation with time. There were no fewer nerve fibres identified in those specimens that had undergone IDET. Specimens obtained six weeks after IDET showed evidence of thermal necrosis in the inner annulus, sparing the periphery of the disc. The reported benefit from IDET appears to be related to factors other than denervation. Thermal necrosis within the annulus six weeks after IDET.

Are peak posterior annular and nuclear temperatures obtained during IDET within the temperature range normally associated with nociceptor destruction and contraction of collagen?

Pain relief following intradiscal electrothermal therapy (IDET) has been reported to result from coagulation of annular nociceptors and contraction of collagen. This requires temperatures respectfully of 45°C and 60°C. A cadaveric study using an intradiscal catheter (Spinecath, Oratec Interventions Inc., CA) reported sufficient temperatures for these events to occur. However a human study reported temperatures sufficient only to coagulate nociceptors. This study reports peak posterior annular and nuclear temperatures attained in-vivo with an intradiscal catheter in sheep.

Twenty sheep were anaesthetised and the lumbar spine exposed. In two non-adjacent discs a stab incision was made in the left postero-lateral annulus and the wound closed. Twelve weeks later the animals returned for a second operation. The spine was approached from the right. Under fluoroscopic control the intradiscal catheter was placed into a previously operated disc. One thermocouple sensor needle was placed 2mm posterior to the catheter to record the posterior annular temperature and a second was inserted 2mm anterior to record the nuclear temperature. The process was repeated for a non-operated control disc. Electrothermal energy was delivered according to the recommended heating protocol.

The target temperature of 90°C at the catheter tip was reached in all cases. Data were tabulated with the mean and standard deviation calculated for each site. There was no significant difference between temperatures reached in the ‘degenerate’ discs and those in the control discs. The mean maximum posterior annular temperature was 63.6°C (range 46.8 to 77.7) and the mean maximum nuclear temperature was 67.8°C (Range 51.1 to 81.2).

Intradiscal electrothermal therapy delivered at 90°C in the sheep consistently heats the posterior annulus and the nucleus to a temperature associated both with coagulation of nociceptors and collagen contraction. These findings may contribute to understanding the mechanism of pain relief following IDET.

Primary disc space infections are thought to occur in children because of the abundant vascularity of the disc prior to skeletal maturity, and while they generally resolve with treatment, little is known about the long-term consequences on the spine.

An ovine model of discitis was used to investigate the effects of discitis on spinal development in the growing sheep. Six-week-old lambs underwent lumbar discography at multiple spinal levels using either radiographic contrast inoculated with Staphylococcus epidermidis (inoculated group) or radiographic contrast only (control group). Plain x-rays of the spines were taken at intervals up to 18 months before the animals were killed and the spines removed for histologic and morphometric analysis.

Discs from animals in the control group were radiologically and histologically normal at all time points, and as expected there was a steady increase in vertebral body and disc dimensions. Although not all inoculated animals showed histologic evidence of discitis, disc abnormalities were evident from an early stage. In particular disc height was significantly reduced from 2 weeks after inoculation and vertebral body dimensions were significantly reduced from one year.

Infection of discs at a young age, whether or not it progresses to discitis, has a significant effect on spinal development.