The histological fate of abdominal dermis–fat grafts implanted into the temporomandibular joint (TMJ) following condylectomy was studied. 21 rabbits underwent left TMJ discectomies and condylectomies; 6 were controls (Group A; no graft used); 15 (Group B) had autogenous abdominal grafts transplanted into the left TMJ. Animals were killed after 4, 12 and 20 weeks. Specimens of the TMJ were histologically and histomorphometrically evaluated. At 4 weeks, fat necrosis was clear in all specimens. The dermis component survived and formed cysts with no necrosis. By 12 weeks, viable fat deposits appeared with no evidence of necrotic fat. At 20 weeks, large amounts of viable fat were present in Group B specimens. Group A had no fat, although the missing condyles regenerated. In the presence of viable fat, Group B showed little condyle regeneration 20 weeks after condylectomy. Non-vascularised fat grafts do not survive transplantation, but stimulate neoadipogenesis. The fate of the dermis component of the graft is independent of the fat component. Fat in the joint space disrupts the regeneration of a new condylar head. Neoadipogensis inhibits growth of new bone and cartilage. This has clinical implications for TMJ ankylosis management and preventing heterotopic bone formation around prosthetic joints.
Autologous fat grafts have been used in reconstructive surgery for over a century . Despite the abundance of adipose tissue that can be easily harvested from multiple sites with minimal morbidity, the results of free fat grafting have been generally disappointing . While the fate of free fat grafts in soft tissue augmentation and contour repair for soft tissue defects has been unpredictable, the same degree of unpredictability is also encountered when fat is used to obliterate bony cavities such as the frontal sinus .
A recent radiological study using magnetic resonance imaging (MRI) showed that non-vascularised dermis–fat grafts not only appear to survive, but the fat component also thrived in significant quantities when transplanted to the human temporomandibular joint (TMJ). This raises interesting questions as to whether the survival and growth of a non-vascularised fat graft is dependent on unique factors found only in certain recipient sites in the body, or whether the addition of dermis to the non-vascularised fat graft facilitates its survival.
Fat grafts alone are difficult to handle and difficult to sculpture to suitable sizes. They also fragment easily when placed in confined spaces. The addition of dermis to the fat greatly facilitates the harvesting of finite quantities of fat tissue and simplifies the sculpting of the fat, which is bound to the dermis. Placement into various cavities is also expedited by the dermis, which acts as a convenient carrier for the fat graft that can be easily orientated and anchored to the surrounding recipient bed when attached to dermis .
The dermis–fat graft was introduced to TMJ surgery by Dimitroulis in 2004 when it was first described as an interpositional material for use in gap arthroplasties for the management of TMJ ankylosis . Since 2000, Dimitroulis has also used autogenous dermis–fat as an interpositional graft in joint cavities following TMJ discectomy . The graft, which is harvested from the periumbilical region of the lower abdomen, was never expected to replace the missing disc but was intended as a soft tissue plug to fill the joint cavity when the disc was removed . In the absence of a disc, the intention was for the dermis–fat graft to provide a physical barrier between the condyle and glenoid fossa to prevent heterogenous bone formation and perhaps also to prevent direct contact between the joint surfaces, to minimise wear and tear on the articular cartilage.
The clinical outcomes of TMJ discectomy with dermis–fat grafting have been favourable , but little is known about the histological fate of the dermis–fat graft within the TMJ, and whether the dermis is essential for the growth and maintenance of the fat graft as suggested in a previous MRI study . Using a rabbit model, this study aims to investigate three issues. First, the survival mechanism of the dermis–fat graft when implanted into the TMJ will be assessed at three time points under light and virtual microscopy. Second, the role, if any, the dermis component of the dermis–fat graft plays in the survival of the fat when implanted into the TMJ will be assessed. Third, what influence the presence of dermis–fat graft material has on the regeneration of mandibular condyles in young adult (3 month old) rabbits will be determined.
Materials and methods
This study was approved by the Animal Ethics Committee at St. Vincent’s Hospital Melbourne in accordance with guidelines published by the National Health and Medical Research Council of Australia governing animal experiments. 21 female New Zealand white rabbits underwent left TMJ discectomies and condylectomies ( Table 1 ). All rabbits were young adult females, 3 months old and at least 2 kg at the time of surgery. Six rabbits were used as controls (Group A) and no graft material was placed in any of the resultant TMJ cavities. The remaining 15 experimental animals (Group B) each had dermis–fat grafts harvested from the peri-umbilical region of the lower abdomen, which were transplanted into the left TMJ cavities following surgery.
|Rabbits 1A, 2A||Control – left TMJ condylectomy, no graft|
|Rabbits 1B–5B||Experimental – left TMJ condylectomy with dermis–fat graft|
|Rabbits 3A, 4A||Control – left TMJ condylectomy, no graft|
|Rabbits 6B–10B||Experimental – left TMJ condylectomy with dermis–fat graft|
|Rabbits 5A, 6A||Control – left TMJ condylectomy, no graft|
|Rabbits 11B–15B||Experimental – left TMJ condylectomy with dermis–fat graft|
Harvesting abdominal donor grafts
The 15 experimental rabbits (Group B) had dermis–fat grafts procured from their lower abdomen ( Fig. 1 ). The autogenous grafts were harvested via a 2 cm × 0.5 cm elliptical incision to a depth of 0.5 cm in the lower abdomen of each rabbit. Only the epidermal layer was carefully removed by sharp dissection from the dermis–fat grafts. The donor site wound was primarily closed with 4/0 vicryl sutures. Each graft (2 cm × 0.5 cm × 0.5 cm = 0.5 cm 3 ) was passively inserted into the left TMJ and the joint capsule securely closed. The greatest cross-sectional area of each graft was 50 mm 2 .
Surgical technique for TMJ
A horizontal skin incision was made from just posterior to the lateral canthus of the eye to just anterior to the external acoustic meatus. The zygomatico-squamosal suture line was exposed and a section of the zygomatic process overlying the TMJ capsule was carefully removed. In the 6 control rabbits (Group A) a left side discectomy using sharp dissection and 5 mm condylectomy using fine bone ronguers was performed and the wound was immediately repaired without any graft. In the 15 Group B rabbits, a left side discectomy using sharp dissection and 5 mm condylectomy using fine bone ronguers was performed and an autogenous piece of dermis–fat graft (0.5 cm 3 ) was passively placed in the resultant surgical cavity without any suture anchorage. The joint capsule was closed and the surgical wounds were repaired in layers with 4/0 vicryl sutures.
The animals were killed using IV sodium pentobarbitone (2 mg/kg) 4, 12 and 20 weeks following surgery ( Table 1 ). The animals were decapitated and conveyed to the histopathology laboratory where the left TMJs were dissected out and placed in formalin. The specimens were decalcified prior to histological sectioning. Coronal sections of each TMJ specimen were prepared for histological evaluation under light microscopy. At least 3 sections, 3 mm apart, from each joint specimen were prepared and stained with haematoxylin–eosin for histological examination under light microscopy. Pertinent findings were recorded using digital photography and histomorphometric analysis was performed with virtual microscopy.
Virtual microscopy (quantitative) analysis
The haematoxylin–eosin stained histological slides were digitally scanned using the ScanScope T3 virtual microscopy slide scanner (Aperio, Vista, CA, USA) and ScanScope Console software v7.00.08.1020 provided the user interface. After all the slides were scanned, the digital images were analysed using the ImageScope(r) software package. The fat was selected using the ‘pen tool’ and the ‘positive pixel count’ algorithm was run on the selected tissue. The colour saturation threshold was calibrated for each group, based on the intensity of the stain of the positive control slide, containing adipose tissue alone, to achieve uniformity in measuring the stain for all sections of that group. The same procedure was repeated for the (non-fat) fibrous and epithelial elements to determine the background stain. The number of positive pixels was divided over the surface area to obtain the number of positive pixels per mm 2 for each slide. This value was subtracted from that of the negative control slides, containing non-fat, fibrous and epithelial elements, to exclude background stain and provide an absolute value of the area of fat present for each slide.
Each TMJ specimen had 3 slices taken 3 mm apart in the coronal plane with the middle section sliced at the centre of the specimen. The results are summarised in Tables 2–4 .
|Percentage proportion of in the TMJ necrotic fat||Percentage proportion of in the TMJ viable fat|
|Summary of mean values of necrotic fat (percentage proportion)|
|4 weeks||Group B||91.9 ± 3.5%|
|12 weeks||Group B||0%|
|20 weeks||Group B||0%|
|Evidence of regenerating condyle||Presence of epidermoid cyst in the TMJ|
|Rabbit 8B||Yes (poor)||Yes|
|Rabbit 10B||Yes (poor)||Yes|
|Rabbit 11B||Yes (poor)||No|
|Temporomandibular joint (mean area of fat tissue measured using the virtual microscope)|
|Group B||31.2 ± 6.3 mm 2 (necrotic fat)|
|Group B||39.5 ± 10.1 mm 2 (viable fat)|
|Group B||98.7 ± 23.6 mm 2 (viable fat)|
|Fat graft at time of implantation|
|Group B||50.0 mm 2|