Thermal epiphysiodesis performed with radio frequency in a porcine model

Microwave Ablation of the Pig Growth Plate: Proof of Concept for Minimally Invasive Epiphysiodesis

BACKGROUND

Different surgical methods for epiphysiodesis of limb length discrepancy (LLD) have been described. Although these methods are variably effective, they are associated with morbidity (pain and limp) and potential complications. Microwave ablation is a less-invasive opportunity to halt growth by selectively destroying the growth plate via thermal energy to treat LLD in children.

QUESTIONS/PURPOSES

In this proof-of-concept study using an in vivo pig model, we asked: (1) What is the durability of response 2 to 4 months after microwave ablation of the tibial growth plate as measured by length and angulation of the tibia via a CT scan? (2) Was articular cartilage maintained as measured by standard histologic staining for articular cartilage viability?

METHODS

To develop an in vivo protocol for microwave ablation, we placed microwave antennas adjacent to the proximal tibia growth plate in the cadaveric hindlimbs of 18 3-month-old pigs. To determine the suitable time, we varied ablation from 90 to 270 seconds at 65-W power settings. After sectioning the tibia, we visually assessed for discoloration (implying growth plate destruction) that included the central growth plate but did not encroach into the epiphysis in a manner that could disrupt the articular surface. Using this information, we then performed microwave ablation on three live female pigs (3.5 to 4 months old) to evaluate physiologic changes and durability of response. A postprocedure MRI was performed to ensure the intervention led to spatial growth plate alterations similar to that seen in cadavers. This was followed by serial CT, which was used to assess the potential effect on local bone and growth until the animals were euthanized 2 to 4 months after the procedure. We analyzed LLD, angular deformity, and bony deformity using CT scans of both tibias. The visibility of articular cartilage was compared with that of the contralateral tibia via standard histologic staining, and growth rates of the proximal tibial growth plate were compared via fluorochrome labeling.

RESULTS

Eighteen cadaveric specimens showed ablation zones across the growth plate without visual damage to the articular surface. The three live pigs did not exhibit changes in gait or require notable pain medication after the procedure. Each animal demonstrated growth plate destruction, expected limb shortening (0.8, 1.2, and 1.5 cm), and bony cavitation around the growth plate. Slight valgus bone angulation (4º, 5º, and 12º) compared with the control tibia was noted. No qualitatively observable articular cartilage damage was encountered from the histologic comparison with the contralateral tibia for articular cartilage thickness and cellular morphology.

CONCLUSION

A microwave antenna placed into a pig's proximal tibia growth plate can slow the growth of the tibia without apparent pain and alteration of gait and function.

CLINICAL RELEVANCE

Further investigation and refinement of our animal model is ongoing and includes shorter ablation times and comparison of dynamic ablation (moving the antennae during the ablation) as well as static ablation of the tibia from a medial and lateral portal. These refinements and planned comparison with standard mechanical growth arrest in our pig model may lead to a similar approach to ablate growth plates in children with LLD.

A Preclinical Pilot Study on the Effects of Thermal Ablation on Lamb Growth Plates

(1) Background: Thermal ablation has been demonstrated to affect the bone growth of osteoid osteoma in adolescents. Growth modulation due to thermal heat in children is conceivable, but has not yet been established. We used lamb extremities as a preclinical model to examine the effect of thermal ablation on growth plates in order to evaluate its potential for axial or longitudinal growth modulation in pediatric patients. (2) Methods: Thermal ablation was performed by electrocautery on eight different growth plates of the legs and distal radii of a stillborn lamb. After treatment, target hits and the physical extent of the growth plate lesions were monitored using micro-computed tomography (micro-CT) and histology. (3) Results: Lesions and their physical extent could be quantified in 75% of the treated extremities. The histological analysis revealed that the disruption of tissue was confined to a small area and the applied heat did not cause the entire growth plate to be disrupted or obviously damaged. (4) Conclusions: Thermal ablation by electrocautery is minimally invasive and can be used for targeted disruption of small areas in growth plates in the animal model. The results suggest that thermal ablation can be developed into a suitable method to influence epiphyseal growth in children.

Closing the growth plate: a review of indications and surgical options

PURPOSE OF REVIEW

The purpose is to review the indications for epiphysiodesis in the treatment of pediatric limb length discrepancies; provide an update on current diagnostic, planning, and surgical techniques; and to introduce possible future alternatives.

RECENT FINDINGS

Retrospective comparative studies have failed to demonstrate superiority of one epiphysiodesis technique over the others. EOS low-dose biplanar X-ray and smartphone growth prediction applications are improving our ability to diagnose and plan treatment for leg length discrepancy. Arthroscopically guided percutaneous epiphysiodesis and radiofrequency ablation are newer techniques that are still under investigation.

SUMMARY

Epiphysiodesis is the treatment of choice for children with predicted leg length discrepancies between 2-5 cm, provided that the physes are open with sufficient growth remaining. The most common epiphysiodesis techniques are performed percutaneously and either ablate the physis with drills/curettes or use metal implants to tether the physis and prevent further growth. Surgical treatment is typically ambulatory in nature, and allows for early return to weight bearing. In modern series, complication rates are less than 10%, with the majority being minor complications. Further high-quality prospective research is needed to determine the optimal epiphysiodesis surgical technique.

Guided growth: mechanism and reversibility of modulation

In paediatric orthopaedics, deformities and discrepancies in length of bones are key problems that commonly need to be addressed in daily practice. An understanding of the physiology behind developing bones is crucial for planning treatment. Modulation of the growing bone can be performed in a number of ways. Here, we discuss the principles and mechanisms behind the techniques. Historically, the first procedures were destructive in their mechanism but reversible techniques were later developed with stapling of the growth plate being the gold standard treatment for decades. It has historically been used for both angular deformities and control of overall bone length. Today, tension band plating has partially overtaken stapling but this technique also carries a risk of complications. The diverging screws in these implants are probably mainly useful for hemiepiphysiodesis. We also discuss new minimally invasive techniques that may become important in future clinical practice.

Physeal histological morphology after thermal epiphysiodesis using radiofrequency ablation

Background

Several treatments have been described for leg length discrepancy. Epiphysiodesis is the most commonly used because of its effectiveness. Thermal epiphysiodesis using radiofrequency ablation (RFA) alters the growth plate morphology without damaging the adjacent articular cartilage; it is a minimally invasive method that has shown excellent results in animal models. This study describes the macro and micro morphology after the procedure.

Materials and methods

Epiphysiodesis using RFA was performed in vivo for 8 min (92–98 °C) at two ablation sites (medial and lateral) in one randomly-selected tibia in eight growing pigs. The contralateral tibia was used as control. After 12 weeks, the pigs were killed and the tibiae harvested. The specimens were studied macroscopically and histology samples were obtained. Physeal morphology, thickness and characteristics were then described.

Results

Macroscopically, the articular cartilage was normal in all the treated tibiae. Microscopically, the physis was detected as a discontinuous line on the treated tibiae while it was continuous in all controls. In the control specimens, the mean thickness of the physis was 625 µm (606–639, SD = 14). All the physeal layers were organized. In the ablated specimens, disorganized layers in a heterogeneous line were observed. Bone bridges were identified at the ablation sites. The central part of the physis looked normal. Next to the bone bridge, the physis was thicker and presented fibrosis. The mean thickness was 820 µm (628–949, SD = 130). No abnormalities in the articular cartilage were observed.

Conclusions

Thermal epiphysiodesis with RFA disrupts the physeal morphology and causes the formation of bone bridges at the ablation sites. This procedure does not damage the adjacent articular cartilage. The damaged tissue, next to the bone bridges, is characterized by disorganization and fibrosis.

Does radiofrequency ablation (RFA) epiphysiodesis affect adjacent joint cartilage?

PURPOSE

To test the hypothesis that epiphysiodesis made with radiofrequency ablation (RFA) is a safe procedure that disrupts the growth plate without damaging the adjacent joint articular cartilage.

METHODS

RFA epiphysiodesis was done during 8 min in vivo in 40 growing pig tibia physis. In addition, three tibiae were ablated for 16 min and three more for 24 min. As a burned cartilage reference, six tibiae were ablated on the joint articular cartilage for 8 min. After the procedure, the animals were terminated and the tibiae were harvested. Magnetic resonance imaging (MRI) was done ex vivo to evaluate the joint articular cartilage in all samples. We used T1-weighted, T2-weighted, and water content sequences under a 1.5 T magnetic field.

RESULTS

On the burned articular cartilage, intensity changes were observed at MRI. We found no evidence of articular cartilage damage on the 40 8-min RFA procedures. The tibiae ablated for 16 min and 24 min showed intact joint cartilage.

CONCLUSIONS

Epiphysiodesis using RFA is safe for the adjacent articular cartilage. This study shows that RFA can be done safely in the growing physis of pigs, even with triple duration procedures.