Mechanical evaluation of a threaded interference interlocking mechanism for angle-stable intramedullary nailing

John Hanlon University of Florida College of Veterinary Medicine, Gainesville, FL

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Stanley E. Kim University of Florida College of Veterinary Medicine, Gainesville, FL

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 BVSc, MS, DACVS

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Abstract

OBJECTIVE

To assess the fatigue and load-to-failure mechanical characteristics of an intramedullary nail with a threaded interference design (TID) in comparison to a commercially available veterinary angle-stable nail with a Morse taper bolt design (I-Loc) of an equivalent size.

METHODS

10 single interlocking screw/bolt constructs of TID and I-Loc implants were assembled using steel pipe segments and placed through 50,000 cycles of simulated, physiologic axial or torsional loading. Entry torque, postfatigue extraction torque, and 10th, 25,000th, and 50,000th cycle torsional toggle were assessed. Each construct was then loaded to failure in the same respective direction as fatigue testing. Four complete constructs of each design were then assessed using a synthetic bone analog with a 50-mm central defect via nondestructive torsional and axial loading followed by axial load to failure.

RESULTS

All constructs were angle stable at all time points and withstood fatigue loading. Median insertional torque, extraction torque-to-insertion torque ratio, and torsional yield load were 33%, 33%, and 72.5% lower, respectively, for the TID interlocking screws. No differences in torsional peak load, torsional stiffness, axial yield load, axial stiffness, or axial peak load were identified. No differences in complete construct angle stability, torsional stiffness, axial peak load, axial stiffness, or axial yield load were identified.

CLINICAL RELEVANCE

The TID had an inferior torsional yield load when compared to I-Loc implants but generated angle stability and sustained simulated physiologic fatigue loading. The TID may be a suitable mechanism for generating angle stability in interlocking nails.

Abstract

OBJECTIVE

To assess the fatigue and load-to-failure mechanical characteristics of an intramedullary nail with a threaded interference design (TID) in comparison to a commercially available veterinary angle-stable nail with a Morse taper bolt design (I-Loc) of an equivalent size.

METHODS

10 single interlocking screw/bolt constructs of TID and I-Loc implants were assembled using steel pipe segments and placed through 50,000 cycles of simulated, physiologic axial or torsional loading. Entry torque, postfatigue extraction torque, and 10th, 25,000th, and 50,000th cycle torsional toggle were assessed. Each construct was then loaded to failure in the same respective direction as fatigue testing. Four complete constructs of each design were then assessed using a synthetic bone analog with a 50-mm central defect via nondestructive torsional and axial loading followed by axial load to failure.

RESULTS

All constructs were angle stable at all time points and withstood fatigue loading. Median insertional torque, extraction torque-to-insertion torque ratio, and torsional yield load were 33%, 33%, and 72.5% lower, respectively, for the TID interlocking screws. No differences in torsional peak load, torsional stiffness, axial yield load, axial stiffness, or axial peak load were identified. No differences in complete construct angle stability, torsional stiffness, axial peak load, axial stiffness, or axial yield load were identified.

CLINICAL RELEVANCE

The TID had an inferior torsional yield load when compared to I-Loc implants but generated angle stability and sustained simulated physiologic fatigue loading. The TID may be a suitable mechanism for generating angle stability in interlocking nails.

Intramedullary nailing is considered the standard of care for long-bone fracture fixation by many human orthopedic surgeons and is now widely accepted in veterinary medicine.16 Interlocking components were first combined with intramedullary rods to improve torsional and compressive forces experienced by the affected bone, but from their clinical introduction in the 1950s until the early 2000s, the interlocking components of intramedullary nails were not designed to rigidly interact with the nail.710 These constructs thus exhibited rotational instability, which is of particular concern as shear forces are especially deleterious to the fracture site healing potential.11,12

Angle-stable interlocking components were designed to generate a rigid connection between fixation elements, such as screws, to connecting elements, such as plates. Angle stability has been successfully applied to intramedullary nails and is effective in eliminating torsional toggle.10,1316 Angle-stable intramedullary nails designed to date typically utilize circumferentially threaded, interlocking components.4,6,13,14,1621 Through the elimination of torsional toggle, angle-stable intramedullary nails have reduced reliance on aggressive intramedullary reaming and may improve the rate and quality of bone healing, improve the fatigue life of the interlocking screws, and potentially reduce the number of necessary screws to achieve a similar mechanical strength when compared to nonangle-stable designs.1822

Although effective in achieving angle stability, currently available designs have drawbacks that affect their clinical applicability or ease of use. Many screw/bolt designs available can be prone to premature cross-threading of components and thereby impairing full engagement and complete seating, damaging implants, or increasing the time and effort for implantation. Several designs have multiple intraoperative steps, such as cutting bolts to length or placing additional inserts, which require specific instrument sets and greater opportunities for technical errors.4,6,17

With the goal of improving the ease and efficiency of implant placement, we developed an interlocking nail with a threaded interference design (TID) (Figure 1), which consists of a partially threaded interlocking screw and partially circumferentially tapped intramedullary nail hole. This design was generated to permit gliding screw insertion to its desired depth and then the creation of a rigid, angle-stable connection with the nail at its desired depth by rotating the interlocking screw 90°. The goal of this study was to evaluate the mechanical characteristics of this novel design in comparison to the current, veterinary standard, Morse taper bolt design (I-Loc) for angle-stable intramedullary nailing of the most similar size. We hypothesized this novel screw and associated intramedullary hole design would perform similarly to the I-Loc design in its ability to achieve and sustain equivalent torsional angle stability and sustain equivalent axial and torsional fatigue and load to failure testing, with similar mechanisms of failure when compared to the I-Loc.

Figure 1
Figure 1

Schematic of the novel, angle-stable, interlocking nail with threaded interference design (TID). The design permits resistance-free sliding of the interlocking screw to the desired depth when in the demonstrated orientation (1 and 2). Once at the desired depth, the screw is rotated 90° (2) to achieve an angle-stable connection between the screw and the nail in its final orientation (3).

Citation: American Journal of Veterinary Research 85, 7; 10.2460/ajvr.24.03.0071

Methods

Implant sourcing and preparation

To assess the I-Loc design, commercially available 6-mm diameter, 147-mm length intramedullary nails with associated 3.2-mm (cis)/2.5-mm (trans) diameter bolts, all of 316L American Society of Testing and Materials (ASTM) F138 stainless steel, were sourced (I-Loc; Biomedtrix). These intramedullary nails consist of an hourglass silhouette with a maximal diameter of 6 mm both proximally and distally and a minimal measured diameter of 4.7 mm at the middle of the nail. Bolts in this testing group were not cut to length for testing. For the TID, intramedullary nails and screws were custom manufactured from 316L ASTM F138-08 stainless steel. The custom nail was created with a similar hourglass design to the commercially available, I-Loc nails with slightly larger proximal and distal maximal diameters of 6.5 mm and a larger minimal diameter of 5.0 mm in the middle of the nails. The novel screws, also of 316L ASTM F138-08 stainless steel, were manufactured at a length of 20 mm and 22 mm with maximal diameters of 3.0 mm (cis) and 2.5 mm (trans). These screws had a core diameter of 2.5 mm throughout the length of the screw and featured partially circumferential threading in the mid region of the screw and smooth, circumferentially unthreaded portions in the regions of the screw that would be placed within cis- or transcortical bone. Holes within the nail for the TID design were drilled to create an oblong hole with a maximum diameter in the sagittal plane of 2.5 mm and a maximal diameter in the transverse plane of 3.0 mm. The hole was tapped such that only the proximal and distal inner faces of the hole were threaded.

Individual screw mechanical assessment

Testing apparatus

To assess only the biomechanical properties of the implants without variability introduced by bone or bone analog deformation, steel pipe segments (19-mm outer diameter X 3-mm wall thickness X 13-mm inner diameter) were utilized for all testing with dimensions reflecting those used in a previous study16 evaluating similar sized implants (Figure 2). Holes of appropriate size for each “cortex” were placed within the steel pipe 13 mm down from the top edge, and, to ensure accuracy and consistency, a drill press along with a custom, 3-D–printed drill guide was utilized for drilling all holes in both test groups. Although steel pipe segments were used for multiple tests, each screw/bolt was tested in a previously unused drill hole. All drill holes were placed within steel pipe segments before testing and were uniformly and sufficiently spaced to reduce variability in load distribution through the steel pipe during testing.

Figure 2
Figure 2

Testing apparatus utilized for all single bolt construct testing (A) and all complete construct mechanical testing (B). Custom, 3-D–printed centralization pieces (black arrows) were used during construction and securement of all constructs within the testing machine and then removed immediately before testing.

Citation: American Journal of Veterinary Research 85, 7; 10.2460/ajvr.24.03.0071

Construct assembly

Intramedullary nails of both testing groups were cut in half to create 2 pieces, both 73 mm in length. When assembling the constructs, centralization of the intramedullary nail component was ensured during screw/bolt placement and securement within the testing machinery using a 3-D–printed centralization piece that was removed before testing. For the purposes of individual screw assessment, constructs consisted of only one screw. All TID screws used for individual screw testing were 22 mm in length.

Insertional torque for each screw/bolt was measured during construct assembly using a commercially available, digital, torque measuring screwdriver (KAIFNT). I-Loc bolts were tightened to a desired torque of 2.4 N·m.22 Once fully inserted into the screw head, the torque necessary to rotate the novel screws 90 into the locked position was recorded. Following insertion, all screws of each test group were assessed for any appreciable deformation or abnormalities.

Biomechanical testing

All mechanical testing was performed using a materials testing machine (MTS Systems Corporation). Force and displacement data were collected at 100 Hz. The intramedullary nail component of the construct was secured to a drill chuck attached to the mechanical testing machine (Figure 2).23 For all axial compression and torsional testing, the steel pipe was secured within a custom apparatus rigidly fixed to the machine load cell. All fatigue tests consisted of 50,000 cycles of loading in either axial compression or torsion in a sinusoidal waveform to simulate 12 weeks of restricted limb use.14,24,25 Five screws/bolts of both designs were tested in axial loading (fatigue then load to failure), while 5 of each design were tested in torsional loading (fatigue then load to failure). Sample size determination was based on a power analysis performed using data collected from preliminary testing. Since each nail section had 2 available holes, nail sections were used for 2 independent, single bolt construct test sequences; however, each bolt/screw was tested using a previously unused intramedullary nail hole of the appropriate design.

Axial compression fatigue testing

Cyclical compression was performed for 50,000 cycles at a rate of 1 Hz under load control from 10 N to 120 N (± 5 N) to simulate the hindlimb weight bearing of a 20-kg dog considering no load sharing between the multiple screws of a complete intramedullary nail construct.8,14 A small load was always present throughout the loading cycle to ensure the construct maintained contact with the base of the testing machine. After fatiguing, the load was completely removed from the apparatus, and screws/bolts were assessed for visual evidence of observable damage, loosening, or other alteration in position. Maximal torque necessary to loosen the screw/bolt was measured, maintaining the central position of the intramedullary nail. Prefatigue insertional torque and postfatigue screw/bolt extraction torques were used to calculate a postfatigue, remaining torque percentage for each sample.

Axial compression load to failure

Following axial fatigue testing and assessment of extraction torque, I-Loc bolts were again tightened to the desired torque of 2.4 N·m, and the TID screws were rotated 90° to the locked position as performed previously. Centralization of constructs was maintained using the previously described centralization piece. Constructs were loaded in compression under displacement control to failure or 10 mm of displacement, whichever occurred first.

Torsional fatigue testing

Torsional fatigue testing was performed for 50,000 cycles at a rate of 1 Hz under load control to ± 0.38 N·m (± 0.05 N·m). Preliminary testing of both screw/bolt designs to ± 0.75 N·m was performed to replicate previous, full construct (4 screws) fatigue testing parameters designed to exceed mid shaft femoral torque of a 20-kg dog, a force previously measured to be approximately 0.6 N·m in a 35-kg dog.14,26 This loading of individual screws, assuming no torsional load sharing between screws, subjectively appeared to generate clinically unrealistic deformation in both groups. To create more realistic, but still likely supraphysiologic, torsional loading, the torque used for torsional fatigue testing of the single screw/bolts was half of that previously reported when testing full constructs and simulating the femoral loading of a 35-kg dog.14,26 After fatiguing, the load was completely removed from the apparatus, and screws/bolts were assessed for visual evidence of observable damage, loosening, or other alteration in position. Maximal torque necessary to loosen the screw/bolt was measured, maintaining the central position of the intramedullary nail. Prefatigue insertional torque and postfatigue screw/bolt extraction torques were used to calculate a postfatigue, remaining torque percentage for each sample.

Torsional load to failure

Following torsional fatigue testing and assessment of extraction torque, I-Loc bolts were again tightened to the desired torque of 2.4 N·m, and TID screws were rotated 90° to the locked position as performed previously. Centralization of the construct was maintained using the previously described centralization piece. Constructs were loaded under displacement control to failure or 20° rotation, whichever occurred first.

Torsional stability assessment

Construct torsional stability was assessed through the generation of torsional load-displacement curves for each screw from the 10th, 25,000th, and 50,000th cycle of torsional fatigue testing. Instability was defined as any region of the load-displacement curve where rotation was observed with a significant decrease in the necessary torque (ie, construct slack).8,10,14

Screw mechanical property assessment

Load-displacement curves were generated for each screw from data collected from load-to-failure testing. For compressive loading, peak load was defined as the maximal load measured within 3 mm of displacement, the point of clinical failure defined for this study. For torsional loading, peak load was defined as the maximal torque measured within 10° of rotation, the point of clinical failure defined for this study. Stiffness was determined for each screw/bolt, defined as the slope of the linear portion of the compressive or torsional load-displacement curve with an R2 of ≥ 0.9. Yield load was recorded for each screw and loading pattern, defined as the point of the load-deformation curve with an appreciable decrease in slope following linear deformation.

Full construct mechanical assessment

Testing apparatus

Preparation of bone model—Cylindrical bone analogs machined from 30% glass-filled structural nylon (Boedeker Plastics Inc) were used for mechanical assessment of full constructs based on previous work14,27 demonstrating this material’s mechanical properties as similar to those of cortical bone. The analog cylinder’s outer diameter was 19 mm with a wall thickness of 3 mm throughout the length of each segment to closely replicate simulated canine femoral constructs previously utilized.16 Analog segments were potted in polyester resin (Bondo; 3M) following orthogonal placement of 2, 0.062-inch Kirschner wires in the portion of the cylinder to be potted to improve model fixation within the resin.14 Analog segments were potted to create a complete construct length of 180 mm including a 50-mm critical bone defect based on previous work assessing similar implant sizes in a simulated canine femur model.16 Following potting, all individual bone segments were left to cure for 24 hours before drilling for interlocking screw/bolt placement using a 3-D–printed drill guide and drill press to ensure accuracy and consistency of drilling. To match previously described screw/bolt dimensions, ciscortex holes were drilled to 3.2 mm in analog segments used for I-Loc constructs and 3.0 mm in analog segments used for constructs made with the new interlocking design. Transcortex screw/bolt holes were drilled to 2.5 mm for all bone analog segments. Four complete constructs were created and tested for each locking mechanism based on the sample sizes and power analyses of similar, previous studies.10,14,28

Full construct assembly—3-D–printed centralization pieces were utilized to ensure the maintenance of nail position within the center of the simulated intramedullary space during the placement of interlocking screws/bolts. These centralization pieces remained in place until completed constructs were secured within the testing apparatus (Figure 2). Each I-Loc bolt was tightened to a torque of 2.4 N·m. I-Loc bolts were not cut to length. For constructs receiving the TID, the proximal and distal most screws 22 mm in length were utilized, while the innermost screws were 20 mm due to screw availability.

Biomechanical assessment—All mechanical testing was performed using a materials testing machine (MTS Systems Corporation). Force and displacement data were collected at 100 Hz. Complete constructs were secured within the mechanical testing machine using a custom securement apparatus (Figure 2).

Nondestructive torsional testing—Full constructs were placed first through nondestructive, torsional testing to ± 2 N·m of torque for 10 cycles at 0.25 Hz. Construct stiffness and torsional stability assessments were performed based on the torsional load-deformation curve generated from the 10th torsional cycle to ensure construct torsional settling before assessment. Stiffness was defined as the slope of the line of best fit applied to the 10th cycle load-deformation curve. Construct torsional instability/slack was defined as appreciable construct rotation without continued, linearly increase or decrease in torque depending on the phase of the loading cycle.8,10,14

Axial compression load to failure—Following previously described torsional loading, constructs were axially loaded from 10 N to 120 N for 10 cycles and then loaded to failure under displacement control to 30 mm of displacement. Load-deformation curves were generated for each construct from load-to-failure testing and used to determine construct axial stiffness, yield load, and peak load. Stiffness and yield load were defined as previously described while peak load for this testing was defined as the maximum load measured within 30 mm of displacement. Deformation and failure characteristics for each construct were evaluated following load to failure.

Statistical analysis

All measured outcomes were compared between test groups using the Wilcoxon rank-sum test. All statistical analysis was performed using commercially available statistical software (SPSS; IBM), and significance was set at P < .05 for all analyses.

Results

Individual screw mechanical assessment

Screw/bolt insertion and extraction torques

The median insertional torque, removal/extraction torque, and remaining torque ratios were 31%, 48%, and 31% lower, respectively, in the TID test group in comparison to the I-Loc group for axial fatigue loading and 35%, 51%, and 35% lower, respectively, for torsional fatigue loading (P < .01) (Table 1).

Table 1

Summary table of screw/bolt torque assessments for single screw/bolt constructs.

TID I-Loc
Axial compression
   Entry torque (N·m) 1.65 (1.1–2.0)* 2.4 (2.39–2.43)*
   Removal torque (N·m) 1.1 (0.6–1.4)* 2.13 (2.01–2.33)*
   Remaining torque ratio 0.61 (0.50–0.74)* 0.89 (0.83–0.96)*
Torsion
   Entry torque (N·m) 1.56 (1.3–2.1)* 2.4 (2.39–2.41)*
   Removal torque (N·m) 1.1 (0.8–1.6)* 2.25 (2.01–2.33)*
   Remaining torque ratio 0.66 (0.63–0.79)* 0.94 (0.83–0.97)*

Values reported as median with associated IQR.

I-Loc = Morse taper bolt design. TID = Threaded interference design.

*

P < .05, different from corresponding I-Loc.

Pre- and postfatigue screw/bolt assessment

No deformation, screw stripping, or cross-threading of any screw/bolt was observed during or following insertional torque application. No deformation, breakage, rotation, or loosening of any screw/bolt in either test group was appreciated during or following axial or torsional fatigue testing.

Torsional stability assessment

No construct torsional instability was appreciated on assessment of the 10th, 25,000th, or 50,000th torsional cycle of any sample in either test group (Figure 3).

Figure 3
Figure 3

Torsional load deformation plot of single screw/bolt constructs during the 50,000th cycle of physiologic torsional loading. I-Loc = Morse taper bolt design. TID = Threaded interference design.

Citation: American Journal of Veterinary Research 85, 7; 10.2460/ajvr.24.03.0071

Screw/bolt mechanical property assessment

No differences were identified between test groups in median axial yield load (P = 1.00), stiffness (P = .84), or peak load (P = .42). The torsional yield load of the TID was 72.5% lower than the torsional yield load of the I-Loc construct (P < .01). No differences were observed between test groups in torsional stiffness (P = .31) or peak load (P = .69) between test groups (Table 2).

Table 2

Summary table for single screw/bolt and full construct mechanical characteristics.

TID I-Loc
Single bolt axial compression
   Axial stiffness (N·m) 4,507.3 (3,622.2–5,503.9) 4,614.8 (4,183.1–5,370.6)
   Axial yield load (N) 2,109.0 (1,850.0–2,472.5) 2,076.0 (1,403.5–2,733.0)
   Axial peak load (N) 3,285.5 (2,566.1–3,763.1) 3,596.8 (2,918.5–4,441.6)
Single bolt torsion
   Torsional stiffness (N·m/°) 1.3 (0.9–1.7) 1.5 (1.4–1.7)
   Torsional yield load (N·m) 1.1 (1.0–1.4)* 4.0 (2.5–5.5)*
   Torsional peak load (N·m) 8.3 (5.9–9.0) 9.5 (5.2–11.2)
Full construct axial compression
   Axial stiffness (N/mm) 1,010.8 (944.0–1,374.9) 1,351.9 (1,239.8–1,394.4)
   Axial yield load (N) 2,108.5 (1,948.2–2,332.2) 2,142.9 (1,942.9–2,319.7)
   Axial peak load (N) 3,907.9 (3,792.5–3,979.9) 3,823.7 (3,756.3–3,908.9)
Full construct torsion
   Torsional stiffness (N·m/°) 0.46 (0.41–0.47) 0.43 (0.42–0.46)

Values reported as median with associated IQR.

I-Loc = Morse taper bolt design. TID = Threaded interference design.

*

P < .05, different from corresponding I-Loc.

Full construct mechanical assessment

Nondestructive torsional assessment

No difference in full construct torsional stiffness was identified between test groups (P = .69). No construct torsional instability was observed in any sample of either test group (Table 2).

Axial compression load to failure

No differences were identified in full construct axial stiffness (P = .34), yield point (P = .89), and peak load (P = .34) between test groups. Consistent failure mechanisms were observed within test groups with different failure mechanisms being observed between test groups. I-Loc constructs failed through a combination of interlocking bolt and nail bending, while TID constructs failed through a combination of interlocking screw bending and subsequent shearing of either the proximal or distal bone analog’s ciscortex (Figure 4).

Figure 4
Figure 4

Representative images of the consistent failure mechanisms observed during axial load-to-failure testing in the full constructs utilizing the threaded interference interlocking nail/screw design, with a partially threaded interlocking screw and intramedullary nail hole (A) and the commercially available I-Loc, with a threaded, Morse taper interlocking bolt design (B).

Citation: American Journal of Veterinary Research 85, 7; 10.2460/ajvr.24.03.0071

Discussion

This study compared the mechanical characteristics of a novel, angle-stable intramedullary nail-screw construct design to those of the current veterinary standard design in angle-stable intramedullary nailing. As hypothesized, both designs, when assessed as single screw/bolt constructs, generated angle-stable constructs with similar fatigue performance when subjected to torsional and axial loading patterns as well as similar mechanical characteristics when destructively loaded in axial compression. The mean torsional yield load was notably lower with TID samples. Although failure mechanisms differed, complete constructs of both designs had similar torsional and axial mechanical properties.

Single screw/bolt construct loading parameters and testing apparatus used in this study were designed to eliminate the variables of screw/bolt load sharing with multiple screws/bolts and mechanical behavior of biologic tissues to assess the implants under a “worst case” loading scenario and create conditions to allow a direct comparison of the bolt-nail interface of the I-Loc and the screw-nail interface of the TID system. Despite the use of presumably supraphysiologic loading parameters, no screws or bolts in either test group or loading condition deformed, broke, or backed out, suggesting both designs can sustain repetitive loading beyond what is typically experienced clinically.

The entry torque of the TID screws was lower than the I-Loc’s Morse taper design in both testing conditions. This result, however, is to be expected as the Morse taper design can be tightened to virtually any torque according to surgical guidelines and surgeon strength, whereas the entry torque of the new design is limited to the maximal torque necessary to rotate the screw 90°. Although the average entry torque of the TID screw was lower than the selected entry torque for the Morse taper design, it closely resembles the recommended entry torque of 1.5 N·m for a similarly sized locking screw (2.7 mm) used with a locking bone plate.29

The percentage of remaining torque for screw/bolt extraction following fatigue loading was lower for the TID than the I-Loc’s Morse taper design for both torsional and axial compressive loading conditions. Angle stability is achieved in the new design through an interference fit that creates mild thread deformation. We suspect this deformation explains lower extraction torque when compared to the initial entry torque. In pilot assessments, the reduced extraction torque was observed immediately following placement without fatigue testing the implant, and because of this, the reduced extraction torque is unlikely to represent screw loosening through repetitive loading.

Although the TID performed similarly as a single screw construct in axial and torsional fatigue assessment as well as the axial load to failure, the TID’s torsional yield load was lower during destructive testing compared to the I-Loc. Given this difference was not observed during axial loading to failure, this reduced yield load is likely related to the oblong geometry of the TID’s intramedullary nail hole. This oblong geometry leaves unoccupied space within the hole adjacent to the flat screw surfaces, which may allow the screw to displace at lower moments under torsional loading. Nevertheless, the TID’s median torsional yield load was 1.1 N·m, nearly twice that of the physiologic torque of 0.6 N·m measured in the mid shaft of a 35-kg dog’s femur via in vivo strain gauge instrumentation during walking.26 The femoral, mid shaft, physiologic torque of a 20-kg dog during walking has not been reported but can safely be presumed to be less in typical circumstances. In addition, this torsional yield load is that of a single screw, whereas load sharing is expected between the 4 screws typically placed in a single construct in clinical subjects.

Complete constructs (an entire nail with 2 bolts/screws in each segment) in bone analog of both designs had similar mechanical characteristics. The axial yield load, stiffness, and maximum load were likely representative of the deformation of the bone analog observed at its interface with the bolt/screw, rather than the implants themselves. This is supported by the similar or even greater destructive axial loading mechanical values measured during testing of the single screw/bolt construct using less deformable, steel pipe. It is important to note that the bone analog material selected for this study was determined previously to closely represent the mechanical characteristics of canine cortical bone under nondestructive conditions,14,27 and therefore, the failure modes of the bone analog may not be clinically realistic. Despite this limitation, the results of complete construct testing in bone analog indicate the ability of both designs to sustain axial force far beyond physiologically expected.

The lack of bending in the new design’s nails and altered failure mechanism is likely multifactorial. The TID’s nail is made of cold-worked, 316L ASTM F138-08 stainless steel, a highly stiff formulation of steel necessary to achieve the desired tolerances of the locking mechanism based on preliminary testing using different steel formulations and processing. This stiffer steel likely improved the nail’s ability to resist the bending observed in the other testing group. The TID’s nail also varied slightly dimensionally from the I-Loc’s nail with a 0.3-mm larger diameter at its narrowest point, generating a larger area moment of inertia and improved resistance to bending forces. For these reasons, the varied failure mechanisms of the full constructs between groups are not believed to be reflective of a clinically relevant flaw in the novel locking mechanism’s design but instead are due to material and dimensional differences of the nails used in this study.

Fatigue testing for this biomechanical assessment utilized a loading protocol based on best evidence to simulate physiologic loading of the canine femur during an activity-restricted, recovery period. However, the true loading of a fractured and then repaired canine femur is not definitively known, and therefore, the loading parameters may not accurately represent loading in a live dog. The loading parameters used also assume uniform loading with each step and isolated torsional and axial forces, where nonuniform and multiplanar loading is expected in the live dog. This study utilized nonbiologic tissues for all biomechanical testing to reduce variability and unrelated influence on the study’s results; however, differences between designs in mechanical characteristics and performance may be appreciable in live subjects for reasons unidentified in this study.

In conclusion, this study sought to assess the biomechanical characteristics of a novel, partially circumferentially threaded screw and intramedullary nail hole design in comparison to a commonly used, commercially available, and angle-stable veterinary design consisting of a Morse taper interlocking bolt. Through assessment of both individual screw/bolt constructs and complete intramedullary nail constructs, the results of this study demonstrate similar fatigue performance, torsion stiffness, and axial biomechanical characteristics of both designs. The median torsional yield load of the I-Loc bolt design was greater than that of the TID screw design; however, both designs’ torsional yield load was greater than the torsional force expected clinically on a single screw/bolt. The results of this study suggest further investigation into the clinical utility of the TID design is warranted.

Acknowledgments

None reported.

Disclosures

The authors have nothing to disclose. No AI-assisted technologies were used in the generation of this manuscript.

Funding

Funding for this project was generously provided through the Edward Debartolo gift.

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