Heat Generation by Two Different Saw Blades Used for Tibial Plateau Leveling Osteotomies

Introduction

Rupture of the cranial cruciate ligament (RCCL) is one of the most common orthopedic diseases encountered in the dog.¹ Surgery is advocated for medium- to large-breed dogs to stabilize the joint and to treat any meniscal injuries that may occur concurrently with RCCL.¹ Currently, tibial plateau leveling osteotomy (TPLO) is one of the more popular RCCL stabilization procedures performed in veterinary medicine.^2,3

The TPLO procedure was developed as a surgical treatment of RCCL in the dog in the early 1990s.³ The originally described procedure involves the use of a specific semicircular biradial oscillating bone saw to create and obtain rotation of a proximal tibial osteotomy and associated tibial plateau. Currently, there are several TPLO biradial and nonbiradial semicircular saw blades commercially available, including the original Slocum biradial saw blade design. The Slocum Enterprises (SE) oscillating TPLO saw blade^a used to create the osteotomy for treating cranial cruciate ligament ruptures in dogs was originally designed as a biradial semicircular saw blade. The blade's unique design was to produce two osteotomy surfaces of perfectly matching radii regardless of the amount of kerf (width of the cut generated by a saw blade) generated by the saw blade's thickness. Recently, several companies, including New Generation Devices (NGD), have developed oscillating TPLO saw blades^b that are concentric and nonbiradial. Minimal kerf is produced by the NGD blade due to a decrease in thickness compared with the SE blade design. Thinner and sharper saw blades have previously been shown to generate less heat than either thicker designs or dull saw blades.⁴

Irrigation is an orthopedic bone osteotomy/ostectomy principle when using a power-based saw. It is recommended by the TPLO saw blade manufacturers to use focused irrigation with sterile fluids to reduce heat generation caused by blade-bone contact during the osteotomy. Currently, it is not known if focused irrigation effectively reduces the heat generated in bone when using a TPLO saw blade.^3,5,6 Other saw blades, drills, and burrs generate heat during osteotomies, with or without irrigation, which may reach the osteonecrosis threshold of 55°C for 1 min.^7–11 Temperatures of this magnitude and duration result in osteonecrosis and may cause osseous resorption and delayed healing of the osteotomy site. Mechanical impairment of bone results at 50°C.⁷ Previous studies have also shown bone regeneration impairment can occur at temperatures as low as 47°C for 1 min.^9,12 Saw blade speed, cutting speed, blade design, as well as irrigation influence heat generation during other orthopedic osteotomy procedures.^6,13–15 Osteotomy temperatures reached may also vary with location within the bone (e.g., depth within thick cortical bone, cis or trans cortex).^16,17

To the authors’ knowledge, heat production during osteotomy of the proximal tibia when performing a TPLO has not been investigated. The authors hypothesized that TPLO saw blade temperatures damaging to bone are reached during the TPLO in canine cadaveric bone. It was also hypothesized that variables such as osteotomy blade design (SE design versus NGD design), cortex involved (cis versus trans cortex), irrigation, and bone size would influence the temperatures generated in the bone and on the TPLO saw blades.

Materials and Methods

Thirty-six paired tibias were harvested from medium- to large mixed-breed dogs (all >20 kg) euthanized for reasons unrelated to this study. All soft tissues were completely dissected from the bone. All tibias were then covered with saline-soaked towels and frozen at −20°C until experimental use. The tibias were thawed at room temperature (19–21°C) for 12–24 hr prior to use. All tibias were kept moist by periodic wetting with saline throughout the study. Each tibia was scanned using a dual-energy X-ray absorptiometry fan beam unit^c (DXA) that allowed for measurement and recording of total bone mineral density (TBD) and total bone area (TBA) for each bone.

Each tibia within a pair was randomly assigned to be either osteotomized using the SE blade or the NGD blade and either receiving irrigation or no irrigation during the osteotomy. The remaining contralateral tibia of the pair was then osteotomized with a different blade than on the ipsilateral tibia of the pair. The same pneumatic saw^d was hand-held for every osteotomy. To reduce interoperator variability and reflect clinical practice, all osteotomies were performed by the same individual (A.B.) using downward pressure and a rotary wrist motion. Irrigation of the saw blades and bone was performed using room temperature saline applied with a surgical bulb syringe.

The distal end of each tibia was securely placed within a vise. The proximal end was instrumented with four thermocouples^e. An appropriate and clinically relevant osteotomy location was chosen on the tibial metaphysis, centered on the intercondylar eminences of the tibia. The TPLO blade assigned to each bone was used to mark and score the tibial metaphysis at the osteotomy site. A 3.2 mm Steinman pin^f was drilled perpendicular to the proximal tibial diaphysis, parallel to the femorotibial joint, and slightly distal to the scored osteotomy site as an alignment guide (Figure 1). For thermocouple placement, two 1.1 mm monocortical holes were drilled in each cortex of the planned proximal tibial fragment: one cranial and one caudal to the Steinman pin and within close and equivalent distance from the curvilinear shape of the planned osteotomy. All holes were within 2 mm of the planned osteotomy and approximately 4 mm from the Steinmann pin in an attempt to maximize temperature conduction from the osteotomy and minimize thermal conduction interference from the pin.⁶ Within each hole, one thermocouple was fitted into the predrilled monocortical hole (Figure 2). Thermocouples were connected to a desktop computer with a special data acquisition board^g and software^h. A fifth thermocouple not fixed to the bone was used to measure each blade's temperature within 2 sec of exiting the trans cortex after completing the osteotomy. The osteotomies were performed with the saw running at full speed. For each thermocouple, a time versus temperature graph was created and analyzed. Temperature acquisition time was set at 5 min or until return to baseline values, whichever was longest, to ensure sufficient time to overcome thermal inertia and to allow cooling of the bone and blade back to baseline readings. For this study, the temperature of interest was the highest temperature recorded for each cortex because it would be the most representative of potentially damaging temperatures reached. Thus, for each set of two cortical graphs, only the graph reaching the highest maximal temperature was considered for analysis. A clinically relevant bone-damaging event was defined as a bone temperature reaching ≥47°C for ≥1 min.^9,12 For each bone cortex, maximal temperature and duration of temperature >47°C were recorded. For each osteotomy, maximal blade exit temperature was also recorded.

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Results

Data for one tibia (one osteotomy) was lost due to a computing error, resulting in a total of 35 osteotomies analyzed in this study. TBD and TBA were used to evaluate homogeneity of bone density and bone size only. The DXA scans revealed a very homogeneous TBD between tibias, with a range of 0.41–0.63 g/cm² (mean±standard deviation was 0.53±0.046 g/cm²). TBA was more variable, with a range of 33.19–59.92 cm² (41.16±7.06 cm²), which reflected a wide range of bone sizes. All osteotomies were completed within 15 sec of their start. There was no association between TBA and saw blade temperature for either saw blade design or irrigated or nonirrigated conditions.

In the cis cortex, the SE blade generated significantly more heat (35±9.1°C) than the NGD blade (31.9±8.54°C) regardless of irrigation status (mean difference was 3.94°C, median difference was 6°C; P=0.0258). There was no significant difference between TPLO blade types with regard to temperature produced in the trans cortex, when considering all cortices together and irrigation status.

The NGD saw blade reached significantly lower temperatures than the SE saw blade (31.9±10.1°C versus 36.76±14.6°C, respectively). Mean difference was 5.94°C and median difference was 2°C (P=0.0034). When analyzing saw blade temperature along with the irrigation status, a significant difference existed only with no irrigation of the osteotomy site and blade (mean difference was 9.78°, median difference was 10°C; P=0.0156) as shown in Figure 3 and Figure 4.

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For the variables saw blade type and cis transcortical location, both bone and TPLO saw blade temperatures were significantly lower with irrigation than with no irrigation (both P<0.05).

Six osteotomies reached bone temperatures ≥47°C (range, 47–88°C). These osteotomies were all performed with no irrigation of the osteotomy site or the TPLO saw blade. Three of these osteotomies were performed with the NGD saw blade and three with the SE saw blade. None of the osteotomies maintained a critical thermal damaging temperature >60 sec. The longest critical thermal exposure duration was 14 sec at ≥47°C. Five of those readings were obtained in the cis cortex and one in the trans cortex, which was consistent with the authors’ findings that the cis cortex heated more than the trans cortex with no irrigation. There were no other significant results noted with any other comparisons considered in this study.

Discussion

This study demonstrates that TPLO saw blade design influences the temperatures reached both within the bone and by the saw blades during the TPLO procedure. Clinical and radiographic union of the TPLO is expected within 8–12 wk. The actual mean TPLO healing time has not been critically evaluated; however, the incidence of complications involving healing after the TPLO procedure is reportedly 18–28%.^18–20 The use of a saw blade to create a TPLO has the potential to produce osseous thermal injury. A subsequent delay in bone healing could prolong osteotomy instability and increase the potential for failure of the implants used for the TPLO procedure. Tissue compromise and necrosis secondary to thermal damage also have the potential to increase the incidence of infections after surgery.

Although there are several TPLO blade designs commercially available, two markedly different TPLO blade designs were selected for this study: the SE saw blade, which has a thicker center with large coarse saw teeth; and the NGD saw blade, which has a thinner center and smaller, finer saw teeth. As expected from previously reported studies examining heat production by surgical saw blades, the robust SE saw blade tended to produce more heat than the finer NGD saw blade.¹² There was no trend toward increasing heat production within this series of osteotomies, regardless of blade design.

Temperature increase differences observed between the two TPLO saw blade designs were greater within the cis cortex than the trans cortex. This may be due to several factors that include the following: (1) heat dissipation by convection from the cadaveric tibial medullary fluid and content present at the time of the osteotomy; (2) any slight variation in trans cortical thermocouple distance to the trans cortical osteotomy; and (3) the duration of the saw blade contact on the cis cortex when starting the osteotomy (contact on the cis cortex is prolonged during initial careful manual alignment of the blade and initiation of the osteotomy).

The influence of irrigation on heat generation during orthopedic osteotomies is well documented and supported by this study.^9,18 Although a basic principle, the use of focused irrigation during osteotomy creation may not be consistent between surgeons when performing a TPLO. In this study, temperatures reached within the bone or saw blade were significantly lower in all cases with irrigation than without. Differences in temperature production related to blade design and cortical location were not statistically significant within the irrigation groups. Critical temperatures and durations (≥47°C for ≥1 min) were not reached in any of the irrigated osteotomies.

The authors considered ≥47°C for ≥1 min as a clinically relevant osseous thermal damage threshold as described in previous studies. In this study, the generation of critical damaging temperatures was uncommon (6/140 measurements) with all temperature peaks lasting <1 min. All critical temperatures occurred only in the absence of saw irrigation.

Thus, against the authors’ expectations, the bone damaging threshold was not reached in any sample. Starting bone temperature was lower than normal canine body temperature and heat dissipation may have been enhanced by the absence of surrounding soft tissues. Subjectively, the osteotomies were performed similarly to those done in a clinical setting, but differences in saw handling and saw advancement speed through the bone may have occurred. Thermocouple readings may have also been affected by the thermocouple tip being press-fitted into the bone rather than linked with thermoconductive paste. Because this study involved cadaveric tissue, postosteotomy histopathologic analysis was not performed. Given the lack of clinically relevant thermal events found in this study, histopathology and in vivo bone healing assessments would better define the true biologic effects of the osteotomy. Although thermal osseous damage could occur during the TPLO, based on this study, the probability appears to be low and is negligible when combined with irrigation.

Bone size (TBA) measured using DXA did not seem to influence heat generation in this study. This may have been due to the thinness of the canine metaphyseal cortices and the resulting potential to produce less frictional contact with the saw blade (i.e., less heat). Another possible explanation for the absence of bone size influence on heat generation during the osteotomy was the potential for a type II statistical error in this study.

All osteotomies were performed with a commercially available hand-held TPLO saw held at full speed by the operator. The authors chose to establish an experimental model as close as possible to the clinical setting when performing the TPLO. Although repeatability and consistency are important when performing a study such as this, the authors believed creating a special jig or similar apparatus to mimic the complex downward and wrist-based rotary motion of the TPLO saw blade around its axis to create the osteotomy would not have produced a clinically relevant experimental study environment to measure heat production. Having all osteotomies performed by one trained individual had the advantage of reducing interindividual variation.

This study had several limitations. First, the sample size of tibias used was limited, which did not allow for more extensive evaluation of blade dulling or bone size influence on heat generation. Periodic sharpening of the blade is recommended by the manufacturers. SE recommends sharpening the SE blade after every use. According to the manufacturer (A. Khowaylo, oral communication, November 2007), recommends sharpening the NGD blade every 25–45 cuts. Neither company's recommendations are based on any published reports. A large number of osteotomies or specific dulling processes would be required to properly assess this variable and were beyond the scope of this study. Dulling, by increasing osteotomy duration and contact surface, has the potential to significantly increase measured osseous and saw blade temperatures. The authors chose not to sharpen the blades during this study considering the low number of osteotomies performed with each blade and to minimize study variables. Second, all bones tested were cadaveric; therefore, the thermal effects of blood flow (especially medullary convection) and surrounding soft tissues could not be assessed in this study. Third, the influence of soft tissues on irrigation efficacy (especially in the trans cortex) could also not be assessed. Soft tissues around the proximal tibia, in a clinical setting, may prevent irrigation from properly reaching all bone surfaces. Fourth, at the start of the osteotomies, the bones used in this study were at room temperature, which may have had a temperature generation dampening effect when creating the TPLO. However, bone temperature of a surgically approached and anesthetized patient is unknown and may be lower than normal body temperature. Although not feasible at the time of this study, it may have been advantageous to assess thermal generation during the creation of the osteotomy while maintaining bone specimens in an incubator at or near 38°C. The thermal kinetics observed in this study (as shown by the short durations of critical temperatures) after completion of the osteotomy suggest that temperatures would be unlikely to remain elevated enough and be prolonged enough to surpass the bone injury threshold reported in the literature even with the test bone temperature raised to a biologically euthermic level. Fifth, although the exact length of time required to create the osteotomies were not recorded, they were kept within a time range of <15 sec. Lastly, the osteotomies were maintained within previously published ranges of distance from thermocouples, there was the potential for small variations of thermocouple distances to the osteotomy site due to the manual use of the saw, which may have affected heat recordings and therefore specimen comparisons for heat production. To keep the osteotomy within adequate distance of the trans cortical thermocouples, a saw blade alignment reference (Steinman pin) different than the standard Slocum jig was used. The standard Slocum alignment jig could not be used in this study due to its interference with the desired position placements of the cis cortical thermocouples near the osteotomy site. The authors considered this to not be an issue because there are a number of surgeons performing the TPLO without said jig. At the authors’ institution, some surgeons perform the procedure using a positive profile pin within the proximal tibia as their only osteotomy alignment device.

Conclusion

This study found that the potential for osseous thermal damage during the creation of a TPLO is low and appears negligible when using saw blade irrigation. None of the experimental groups generated heat likely to cause clinically relevant thermal damage, but TPLO blade design was critical in heat generation because a thinner blade with smaller, sharp, fine teeth (NGD blade) generated lower temperatures in canine cadaveric tibias than a thicker blade with larger, sharp, coarse teeth (SE blade). Irrigation was very efficient in limiting bone and blade temperature elevations and consistently kept temperatures below all osseous thermal damaging levels. The use of the NGD blade in combination with irrigation successfully minimized temperature changes in this study. Several thinner TPLO blade designs are currently available and these may also limit the potential for bone thermal necrosis. Future studies investigation the influence of blade dulling and heat generation in vitro and in vivo during TPLO with different TPLO saw blade designs are warranted.