Materials and Methods

Sample population—Thirty pairs of MC3s were harvested from adult horses (range in body weight, 450 to 550 kg) that had been euthanatized for reasons other than orthopedic disease. All soft tissues were removed, and the metacarpal bones were wrapped in towels soaked in physiologic saline (0.9% NaCl) solution. The bones were then stored at −20°C until testing. Prior to pin insertion, the bones were thawed slowly to room temperature (20°C) for 24 hours.

Pin insertion—One MC3 of each pair was assigned to the STTP group (n = 30); the other was assigned to the NTTP group (30; Figures 1 and 2). All pins were inserted in a lateral to medial direction. For pullout testing, 1 pin was inserted into the mid diaphysis of each bone (n = 20 bones/group) at half length between the proximal end and the distal end of the MC3. For microstructural evaluation, 2 pins were inserted into the distal portion of the diaphysis of each bone (n = 10 bones/group). The first pin was inserted at the distal fourth of the length of the MC3, and the second pin was inserted 3 cm proximal to the first pin. To ensure correct placement of the pins through the medullary cavity, pins were inserted at the lateralmost aspect of the MC3, at half the distance between the dorsal and palmar aspects of the cortex as measured with calipers and with the aid of a laser level to maintain a perpendicular angle to the MC3.

View Full Size

Photograph of an STTP (A) and an NTTP (B) with corresponding tap (C).

Citation: American Journal of Veterinary Research 71, 8; 10.2460/ajvr.71.8.976

• Download Figure
• Download figure as PowerPoint slide

View Full Size

Magnified photograph of the threads and cutting flute of an STTP (A) and NTTP (B).

Citation: American Journal of Veterinary Research 71, 8; 10.2460/ajvr.71.8.976

• Download Figure
• Download figure as PowerPoint slide

To control drill speed and drill force during bone penetration, a drill press^b was used at 720 rpm and with a force of 60 N to advance the lever arm. Sequential drilling was used for creation of the pilot hole, starting with a 4.5-mm drill bit, followed by a 5.5- and a 6.2-mm drill bit. In the STTP group, pins^c were inserted after creation of the pilot hole, whereas in the NTTP group, the pilot hole was pretapped prior to pin^d insertion. Irrigation at room temperature with saline solution (150 mL/min) was provided during drilling, tapping, and pin placement at the cis cortex. Irrigation was stopped while the drill bits were changed. The trans cortex was shielded by attaching to the dorsal and palmar cortex a thick plastic membrane with silicone glue; in this way, lavage fluid could only reach the trans cortex via the drill hole while drilling, to mimic surgical conditions. All testing was performed with the MC3 bones at room temperature (21°C).

Temperature measurement—Bone temperature was measured during insertion of each pin (n = 40 pins/ group). For temperature measurements, one 0.27-mm-diameter Teflon-insulated thermocouple^e was placed on the cis and trans cortices at 1 mm from the final pin thread position in predrilled holes (1 mm in diameter; 2 mm in depth). The thermocouples were coated with thermoconductivity paste^f and placed in their wells on the cis and trans cortices (Figure 3). Temperatures were continuously recorded with a data acquisition device^g connected to a personal computer (1 measurement/s) during drilling, tapping, and pin placement, and the recording was continued for 3 minutes after the pin placement. The ΔT during pin insertion and the duration of temperature increase > 10°C were determined from the recorded data.

View Full Size

Photograph showing thermocouples installed on cis (*) and trans (†) cortices at 1 mm from final thread position in an equine MC3 to measure bone temperature after insertion of a fixation pin or pins.

Citation: American Journal of Veterinary Research 71, 8; 10.2460/ajvr.71.8.976

• Download Figure
• Download figure as PowerPoint slide

Cyclic loading—Fifteen pairs of the bone-pin constructs were loaded in a custom-designed device 1,000 times from 0 to 3,000 N at a rate of 2 Hz by use of a haversine function of the material testing machine.^h Bones were loaded from the proximal end while supported distally by the transfixation pins, simulating in vivo loading. Following cyclic loading, pullout testing or microstructural analysis was performed.

Pullout testing—Pullout resistance was tested in 10 MC3 pairs before and in 10 MC3 pairs after cyclic loading. The bone-pin construct was mounted into a custom-designed jig for retrograde axial extraction of the transfixation pin in a material testing machine.^h Single load to failure was applied with a crosshead displacement of 0.3 mm/s. Tensile force at failure (bone breakage or pin slippage) and the load displacement curve of each specimen were recorded and evaluated with a customized computer program.ⁱ After failure, the total bone width and the width of both cortices were measured at the pin insertion site with calipers. Total cortical thickness was calculated by summation of lateral and medial cortical widths. In bone-pin constructs that did not fail, the pin was reversed and the bone was cut with a band saw through the pin hole. Holding power (N/mm) of the bone-pin construct was calculated by dividing the ultimate pullout resistance by the total cortical width for HPCW and by the total bone width for HPBW.

Scanning electron microscopy—Scanning electron microscopic evaluation of the bone-pin interface was performed on 10 pairs of MC3s (5 pairs before cyclic loading and 5 pairs after cyclic loading). Two pins were inserted in each bone specimen at the distal fourth of the total bone length and 3 cm proximal to the first pin to mimic surgical practice. Specimens were cut with a diamond band saw^j through the long axis of the transfixation pin. The transfixation pin was then lifted carefully from the bone, the cis and trans cortices were identified, and the bone was stored in 70% isopropyl alcohol. The samples were then dehydrated with increasing concentrations of alcohol (30%, 50%, 70%, 80%, 90%, 100%, 100%, and 100%). Afterward, the specimens were dried to the critical point with liquid carbon dioxide for 1 hour, mounted on stubs, and dried overnight in a desiccator. The specimens were then sputter-coated with gold-palladium, and the surfaces were evaluated and photographed with a scanning electron microscope.^k Micrographs of cis and trans cortices were generated at a magnification ranging from 10X to 40X. Microfractures were scored by 2 examiners who were unaware of the treatment group, as follows: 1 = nondisplaced and ≤ 2 mm long; 2 = nondisplaced and > 2 mm long; and 3 = displaced.¹⁴ Scores for each section were added, and a mean score for cis and trans cortices of each pin before and after cyclic loading was determined.

Statistical analysis—A paired t test was used for comparison of the mean ΔT and duration of temperature increase > 10°C as well as for pullout resistance of non–cyclic- and cyclic-loaded bone-pin constructs between the STTP and NTTP groups. To compare holding power before and after cyclic loading as well as cortical thickness between bone-pin constructs that failed and did not fail, a t test for independent samples was performed. A paired t test was used to compare microfracture scores between groups and a t test for independent samples for comparison of microfracture scores between the 2 examiners. A computer program^l was used for all analyses, and differences were considered significant at a value of P < 0.05. Results are reported as mean ± SD.

Results

Temperature measurements—In the STTP and NTTP groups, 5 and 3 pin insertions had to be excluded, respectively, because of unreadable temperature curves. Mean ΔT was significantly higher during insertion of the STTP than during taping and insertion of the NTTP at cis and trans cortices (P < 0.001; Table 1). The number of inserted pins that reached ΔT > 10° was higher in the STTP group (35/35) than in the NTTP group (tap 26/37; pin 13/37). No significant difference was found within each group between ΔT increase at cis and trans cortices.

The duration of ΔT > 10°C during pin insertion was longer in the STTP group than in the NTTP group but not significantly different. The duration of ΔT > 10°C was significantly (P < 0.001) longer when STTP insertion was compared with tapping of the pilot hole in the NTTP group. When duration of temperature increases > 10°C during pin insertion was compared between the pin groups, the increase was significantly (P < 0.001) longer at the trans cortices than at the cis cortices.

Pullout resistance—No significant difference was found when pullout resistance was compared between the STTP and NTTP groups before (P = 0.78) and after (P = 0.64) cyclic loading (Table 2). The calculated bone and cortical holding power were not significantly different before (P = 0.79 and P = 0.59, respectively) or after (P = 0.63 and P = 0.77, respectively) cyclic loading between the STTP and NTTP groups. The total cortical thickness was significantly (P = 0.002) greater in bonepin constructs that did not fail (mean ± SD thickness, 24.6 ± 1.7 mm) than those that did fail (22.2 ± 2.7 mm). In the non–cyclic-loaded group, 6 of 10 pairs of bone-pin constructs failed, and in the cyclic-loaded group, 5 of 10 pairs of bone-pin constructs failed. All constructs that failed, regardless of the group, did so by developing a transverse fracture through the pin hole.

Scanning electron microscopy—The mean score of both examiners for evaluation of microfractures was significantly lower in the STTP group than in the NTTP group (examiner 1: score for STTP was 2.7 ± 3.4 vs 6.0 ± 3.8 for NTTP [P = 0.02]; examiner 2: score for STTP was 3.8 ± 2.1 vs 7.0 ± 3.6 for NTTP [P = 0.01]). No statistical difference was found between the examiners (P = 0.44 for STTP and P = 0.24 for NTTP). Debris was seen in all scanning electron micrographs in the STTP group but only in 41% (50/122) of all images in the NTTP group (Figure 4).

View Full Size

Scanning electron micrographic images comparing STTPs (A, C, and E) and NTTPs (B, D, and F) that had been inserted into MC3s of adult equine cadavers. Images reveal clean threads (A and B), bone debris covering the threads (C and D), and microfractures to threads (E and F). Bar = 0.5 mm.

Citation: American Journal of Veterinary Research 71, 8; 10.2460/ajvr.71.8.976

• Download Figure
• Download figure as PowerPoint slide

Discussion

Excessive heat generation during drilling of the pilot hole and insertion of the transfixation pin is an important factor that can lead to bone necrosis and subsequent premature pin loosening when using the transfixation pin-casting technique in horses.^5,15,16 The threshold temperature for bone necrosis is believed to be 56°C because alkaline phosphatase denatures rapidly when exposed to this temperature.¹⁷ Histochemical evidence of osteocyte death was detected adjacent to an implanted scald that was heated to 50°C for 30 seconds.¹⁸ The same finding was evident in femurs of rabbits in an in vivo study¹⁹ in which a bone temperature of 47°C was sustained for 1 minute. That study also showed that with every temperature increase of 1°C higher than 47°C, the exposure time required to cause bone necrosis is reduced in half. The high temperature did not result in acute bone necrosis but in replacement of bone with fat cells 3 weeks after the insult.

In our study, we measured temperature increase in bones at room temperature (21°C) rather than at a physiologic temperature (37°C) to avoid the variable of bone cool-down while performing the testing in a room-temperature environment. Physical characteristics are reportedly equal at a room temperature of 21°C and at 37°C, except for the elasticity of bone, which increases slightly at body temperature.²⁰ Also, models to calculate heat generation during drilling of bone reveal that bone temperature has only small influence on total heat generation during drilling.^21,22 During insertion of the STTP, temperature increases were significantly (P < 0.001) higher and the duration of the temperature increase was longer than during insertion of the NTTP, although the difference in durations was not significant (P = 0.18). Higher temperature generation during insertion of the STTP versus the NTTP may have been caused by insufficient debris removal during insertion. As the pin cuts its own threads into the bone, the STTP temperature increases during insertion and heat will be stored within the pin and the adjacent bone; therefore, the bone-pin interface will be exposed for a longer period to higher temperatures than if an NTTP was used, which may result in bone necrosis. Similar observations were made in another study¹² in which a self-drilling and self-tapping transfixation pin was reportedly much warmer than the NTTP when the hardware temperature was measured after insertion (72.73 ± 16.92°C vs 42.53 ± 9.59°C, respectively).

Bone is a poor heat conductor; therefore, heat will accumulate at a drilling site and cannot be easily eliminated other than by direct cooling with lavage fluid. This phenomenon has been also noticed after inserting precooled pins into sheep tibiae, in which precooling of the pins has a positive effect at the cis cortex but the precooling effect is lost during insertion and at the trans cortex.¹⁵ By creating threads in a different, additional step, the surgery time may increase but clearance of bone debris in the present study seemed to be easier with the 2 long flutes in the NTTP tap. Thus, exposure to higher temperatures at the bone-pin interface is shorter because the tap is removed after creation of the threads.

Pin loosening can also result from cyclic loading at the bone-pin interface. Forces that frequently change direction and magnitude result in local bone resorption in areas that have contact with the pin. Bone tissue is then slowly replaced with fibrous tissue, which is more tolerant of fluctuating forces that produce deformation in tissues around the implant.^23,24 To avoid increased pin loosening caused by osteolysis, it is important that the implant be well anchored in the bone. Both transfixation pins had similar pullout strength in the present study, and failure of the bone-pin construct was only evident by fracture through the pin hole and not by sharing of the threads. This leads to the conclusion that a strong bone-pin construct was secured for the short term. Neither heat damage nor the caused microfractures appeared to have an effect on the immediate holding power as was evident in other cadaver studies^12,14 of equine bone. Cyclic-loaded bone-pin constructs failed in a similar fashion. Bone-pin constructs were loaded for 1,000 times from 0 to 3,000 N. Other researchers did not find any difference between non–cyclic-loaded and cyclic-loaded bone-pin constructs in their studies^12,14 either. In those studies, loads from 0 to 2,225 N were used for cyclic loading; such loads correspond to 83% of the load expected at a walk in a 454-kg horse. The researchers suggested increasing the load from the 2,225 N used in their studies to 2,670 N, which would correspond to 100% loading at a walk or to increase the number of cycles. In our study, the load was increased to 3,000 N; therefore, the number of cycles may need to be increased to detect a difference between cyclic-loaded and non–cyclic-loaded specimens. On the other hand, cyclic loading in vivo can cause osteoclastic activity and therefore loosening of pins weeks after insertion. In the present study, the holding power of all the pins was tested by retrograde pullout as described for most studies^{5,10,12,14,25} in which pullout strength has been evaluated. Because of the buttress configuration of the STTP, there might be a difference between holding power during retrograde and antegrade pullout that was not investigated and might be of interest for future studies. To finally compare the holding power over time, the pins should be also compared in an in vivo model.

During the present study, only 6 of 10 paired bonepin constructs failed in the non–cyclic-loaded group and 5 of 10 pairs failed in the cyclic-loaded group. As found in previous pullout pin studies,^12,14 the cortical thickness is significantly larger in constructs that did not fail. When the same loading rate (0.3 mm/s) was used, the ultimate tensile strength in this study was lower than that in the previous studies and fewer bonepin constructs failed. But bone-pin constructs that did fail in the present study had larger cortical width (22.2 ± 1.7 mm vs 19.75 ± 2.78 mm and 20.4 ± 2.0 mm) than in those studies.^12,14

Scanning electron microscopic examination of the bone-pin interface in the study revealed significantly (P = 0.02) more microfracture damage in the NTTP group than in the STTP group. The thread of the NTTP is a symmetric V design with relatively wide threads. In another study,¹⁴ microfracturing at the bone-pin interface occurred during tapping of the pilot hole and remained similar after pin insertion.¹⁴ Therefore, the tap of the NTTP may have insufficient cutting abilities for the hard equine bone or it may be that the wide and symmetric thread design causes too much friction. Subjectively, it was noticed during the present study that the NTTP tap became stuck several times during tapping of the pilot hole. Insertional torque was not measured during this study but should be considered for future investigations.

During the microscopic examination of the bonepin interface, bone debris was detected in 41% of NTTP images versus in 100% of STTP images. This finding leads to the assumption that the cutting or tapping flutes of the STTP might be too short so bone debris cannot be sufficiently eliminated during insertion of the pin. Similar findings were reported after insertion of self-tapping Schanz pins into sheep tibiae in vivo.²³ The Schanz pins have the same trocar tip and thread design as the STTP but no real cutting flute. In the present study, it was observed that bone debris was transported into the medullary cavity and to the trans cortical periosteum. Bone debris was only compressed and displaced in a centrifugal direction instead of removing it during pin insertion. In a canine in vitro study²⁵ in which partially threaded pins were used, debris accumulation was noticed at the cis cortex and was blamed for reduced pullout strength relative to the pullout strength at the trans cortex. Pullout strength in our study did not appear affected by the debris accumulation, but the debris may also have contributed to an increase in temperature generation during insertion of the STTP through an increase in friction.

In the study reported here, the new STTP had comparable holding power to the NTTP and caused less microfracture formation at the bone-pin interface. The increased heat generation relative to that of NTTP insertion might lead to bone necrosis and is considered a serious disadvantage of STTP insertion. Additional studies are needed to evaluate whether altering the self-tapping characteristics of the STTP (such as by using longer tapping flutes) or changing the location of the tapping threads within the pin could result in less debris accumulation and heat generation at the drill site.