Biomechanical Comparison of Spacer Pin Fixation to Two Established Methods of Tibial Tuberosity Transposition Stabilization in Dogs

Abstract

Objective The aim of this cadaveric study was to compare the biomechanical outcomes of three methods of stabilization for tibial tuberosity transposition to treat medial patellar luxation: a complete osteotomy with a two-pin and tension band wire (TBW) fixation (TBW group), a partial osteotomy with a two-pin fixation (2 Pin group), and a partial osteotomy with a spacer pin fixation (Spacer Pin group).

Study Design Thirty medium to large-sized canine cadaveric tibiae were dissected and randomly assigned to one of three groups: TBW, 2 Pin, and Spacer Pin groups. The patellar ligaments were loaded in tension until ultimate failure. Ultimate failure force and mode of failure were documented, stiffness was calculated, and the results were compared statistically between the three treatment groups.

Results There were not any significant differences in ultimate failure force or stiffness between groups. All groups predominantly failed by patellar ligament failure, with distal tibial crest fracture/displacement being the second-most common mode in the 2 Pin and Spacer Pin groups.

Conclusion The mechanical properties of the spacer pin stabilization were not different from the TBW and 2 Pin groups. The spacer pin technique could be an alternative way to stabilize tibial tuberosity following tibial tuberosity transposition with a partial osteotomy based on this cadaveric load-to-failure model.

 

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Keywords

biomechanics - orthopaedic surgery - patellar luxation - tibial tuberosity transposition - spacer pin technique

Introduction

Medial patellar luxation (MPL) is a common pathology that causes lameness in dogs.[1] [2] [3] [4] Tibial tuberosity transposition (TTT) is used to realign the quadriceps mechanism as part of the surgical treatment for MPL.[5] The tuberosity segment is stabilized using implants such as pins,[6] [7] [8] wires,[9] [10] [11] plates,[10] [11] and/or screws[12] [13] through or medial to the osteotomized segment, and pins may be combined with a tension band wire (TBW).[6] [7] [8] [14]

Surgical intervention to treat MPL has a postoperative complication rate ranging from 18 to 43%.[1] [3] [15] [16] [17] [18] The most commonly reported soft tissue complications include dehiscence, infection, soft tissue irritation, seroma formation, and patellar desmitis, and the most commonly reported implant or bone-related complications include implant failure, tibial tuberosity avulsion or fracture, and patellar reluxation.[1] [3] [15] [16] [17] [18] While TTT is reported to reduce reluxation rate, many complications can be attributed to a pin being placed through the tuberosity, or to the use of a TBW for stabilization of the TTT.[15] [16] [17] [18] When performing a TTT using a standard osteotomy and tuberosity pin fixation, increased complication rates were associated with the use of a complete osteotomy, using only one pin through the tuberosity (without TBW), and placing the pin(s) in a caudodistal direction.[15] [16] In an attempt to mitigate the risk of complications such as soft tissue irritation, tuberosity fracture, and tuberosity avulsion, the use of a partial osteotomy and “spacer pin fixation” has been utilized to stabilize tibial tuberosity.[19] There have been no mechanical studies on the spacer pin fixation and no studies comparing the spacer pin technique to established TTT stabilization methods.

This cadaveric study compared the biomechanical outcomes of TTT performed by a complete osteotomy with a traditional pin and TBW fixation (TBW), partial osteotomy with a tuberosity pin fixation (2 Pin), or partial osteotomy with a spacer pin adjacent to the tibial tuberosity (Spacer Pin), loaded in tension. The study's aims were to compare (1) the mechanical properties (ultimate failure force and stiffness) and (2) the mode of failure between stabilization methods when loaded in tension in an ex vivo study. We hypothesized that the mechanical properties of the Spacer Pin group would not be different from the 2 Pin and TBW groups. Secondarily, we hypothesized that the Spacer Pin group would fail by distal crest fracture, the 2 Pin group would fail by tuberosity segment fracture at the level of either the pins or distal crest, and the TBW group would fail by patellar ligament failure.

 

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Methods

A sample size of 10 tibias per group was chosen based on prior studies.[7] [8] Thirty medium to large breed skeletally mature canine hindlimbs (NASCO Education, Fort Atkinson, Wisconsin, United States) were used in this study. All limbs were shipped frozen and stored at –18°C. The freeze–thaw cycles were standardized across samples, and tissue moisture was maintained by wrapping with saline-soaked gauze. All muscles and soft tissue structures were removed from the tibia and fibula except the patellar ligament, patella, and distal quadriceps complex. Tibial length (cranial tibial plateau to distal medial malleolus) was recorded. Using randomization along with tibial length and sidedness matching between groups, limbs were assigned to receive a complete osteotomy with a two-pin and TBW fixation (TBW group), a partial osteotomy with a two-pin fixation (2 Pin group), or a partial osteotomy with spacer pin fixation (Spacer Pin group; [Fig. 1]).

Fig. 1 Surgical procedures. Cadaveric canine hindlimbs (N = 30) were dissected and assigned one of the three surgical techniques for tibial tuberosity transposition stabilization: (A) a complete osteotomy with a two-pin and tension band wire fixation (TBW group, n = 10), (B) a partial osteotomy with a two-pin fixation (2 Pin group, n = 10), and (C) a partial osteotomy with a spacer pin fixation (Spacer Pin group, n = 9).

Surgical Procedure

Osteotomy Creation and Transposition

The osteotomy was standardized by mounting each tibia into a jig that allowed modified use of a tibial tuberosity cutting guide (OssAbility, Christchurch, Canterbury, New Zealand). The wooden jig consisted of two vertical 2.5-mm pins (IMEX Veterinary, Longview, Texas, United States) that allowed the cutting guide to slide down over the medial aspect of the bone such that the frontal plane osteotomy would be directed between[1] the midpoint between the tibial tuberosity and the cranial aspect of the tibial plateau and[2] the midpoint of the slope between the lesser tibial tuberosity and the straight tibial diaphysis ([Figs. 2] and [3A]). The cutting guide was secured to the bone proximally, just caudal to the osteotomy, with one 0.9-mm Kirschner wire (Securos Surgical, Sturbridge, Massachusetts, United States). An incomplete linear osteotomy was made using an oscillating saw (Veterinary Surgical Solutions, Austin, Texas, United States) (no. 11 blade, 0.5-mm thickness), leaving the distal crest intact ([Fig 3B]). Of the projected osteotomy, the lateral aspect of the osteotomy was completed for the proximal 60% and the medial aspect of the osteotomy was completed for the proximal 80%, and the distal 40% (lateral) and 20% (medial) of the projected osteotomy remained intact ([Figs. 2] and [3C]).

Fig. 2 Tibial tuberosity osteotomy landmarks. The tibial tuberosity osteotomy began at the midpoint between the tibial tuberosity and the cranial aspect of the tibial plateau (point A). The osteotomy extended vertically from the proximal tibial landmark (point A) toward the midpoint between the lesser tibial tuberosity and vertical tibial diaphysis, with the osteotomy only completed 60% of this distance laterally (point B) and 80% of this distance medially (point C). The distal tibial crest was left intact.

Fig. 3 Use of OssAbility tibial tuberosity osteotomy guide and Tibial Tuberosity Transposition Tool to create the osteotomy and transpose the tibial tuberosity. (A) Osteotomy position was determined, and osteotomy length measured; 60 and 80% of the length of the full osteotomy was calculated. (B) The bone was secured to the osteotomy jig, the osteotomy guide was placed, and the osteotomy was initiated. (C) After guide removal, the osteotomy was completed to 60% of the tibial crest on the lateral cortex and 80% on the medial cortex. (D) The sample was moved to the transposition jig and the tuberosity was translocated laterally at 0.7 mm per minute using the Tibial Tuberosity Transposition Tool. (E) Transposition of 4 mm was confirmed and (F) the assigned fixation technique was performed.

The tibia was moved to the transposition jig, which allowed modified use of the Tibial Tuberosity Transposition Tool (TTTT, Clinica Veterinaria Milano Sud, Milan, Italy). The proximal tibia was positioned between two 2.5-mm pins (IMEX Veterinary) angled approximately 25 degrees from cranioproximal to caudodistal (approximately parallel to the tibial plateau), and 70 degrees divergent from each other in the transverse plane ([Fig. 3D]). The TTTT was assembled onto the pins, with the paddle against the medial aspect of the tibial tuberosity. The TTTT was used to slowly transpose the tuberosity laterally at a rate of 0.7 mm/min (1 turn/min; [Fig. 3D]) until 4 mm of transposition was achieved at the level of the tibial tuberosity, confirmed with a ruler ([Fig. 3E]). The magnitude of transposition was based on pilot data where it was determined that this distance would represent approximately one-fourth to one-third of the width of the tibial crest, which allowed the use of the same size spacer pin across specimens. The randomly assigned stabilization technique was then performed ([Fig. 3F]).

 

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Complete Osteotomy with Two Pins and Tension Band Wire (TBW Group)

Two 1.6-mm Kirschner wires (IMEX Veterinary) were placed adjacent to each other in the transverse plane starting approximately 1 mm proximal to the distal aspect of the insertion of the patellar ligament, and exiting the caudomedial cortex of the tibia just distal to the joint. Once the tuberosity was stabilized with two Kirschner wires, the osteotomy was completed through the distal tuberosity segment to exit the tibial cortex. A bone tunnel was created approximately 5 mm caudal and just distal to the distal aspect of the osteotomy, perpendicular to the bone's long axis, using a 1.6-mm Kirschner wire (IMEX Veterinary). An approximate 60-mm length of 1.0-mm stainless steel orthopaedic wire (Securos Surgical) was used to create a figure-of-8 TBW around the two pins and through the bone tunnel. The twist was bent caudally during final tightening, then the wire was cut to retain a minimum of three twists on the medial tibia. The Kirschner wires were bent proximally using the double-bend method with the fulcrum 30 to 40 mm from the bone entry point, with the ends facing proximolateral and proximomedial and cut at 3 mm[5] ([Fig. 1A]).

 

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Partial Osteotomy with Two Pins (2 Pin Group)

In the 2 Pin group, the tibial tuberosity was stabilized with two 1.6-mm Kirschner wires (IMEX Veterinary), while the TTTT maintained the transposition, similar to the TBW group except that the osteotomy remained incomplete and no TBW was placed ([Fig. 1B]).

 

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Partial Osteotomy with Spacer Pin (Spacer Pin Group)

In the Spacer Pin group, a “spacer pin” (2.5 mm, IMEX Veterinary) was inserted medial to the tuberosity segment at the level of the tibial tuberosity and exited the caudomedial cortex just distal to the joint, while the TTTT maintained the TTT. The spacer pin was cut flush with the tibial tuberosity ([Fig. 1C]).

 

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Biomechanical Testing

For all groups, postoperative lateral view radiographs (Pausch, Model B-150H, Tinton Falls, New Jersey, United States) were performed to confirm that no fractures occurred and to assess implant positioning ([Fig. 1]). Tibiae were truncated by distal transverse osteotomy such that a 60-mm length of tibia would be within the potting material and the length of the exposed tibia would be equal to twice the length of the tibial crest (measured from greater to lesser tibial tuberosities). Two monocortical screws (19-mm length, 1.6-mm diameter, Teks, Glenview, Illinois, United States) were partially seated within the distal tibia to increase pullout resistance of the bone from the potting material (Bondo Fiberglass Resin, 3M, St Paul, Minnesota, United States). During potting (cylindrical plastic tubing, 101.6-mm outer diameter, 3.2-mm thickness, McMaster-Carr, Elmhurst, Illinois, United States), perpendicularity of the long axis of the tibia to the base of the pot was ensured using two orthogonal laser levels. The tibia was positioned such that the cranial laser level marked the medial-to-lateral center of the distal half of the exposed tibial diaphysis and the medial aspect of the transposed tuberosity at the level of the tuberosity prominence, and the medial laser level marked the center of the distal half of the exposed tibial diaphysis and intersected the proximal tibia at the cranial aspect of the tibial plateau. In the material testing machine (Material Testing System, Model 858 Biaxial, Eden Prairie, Minnesota, United States), the pots were secured into a custom fixture, which allowed the sagittal plane long axis of the tibia to be set at 135 degrees from the patellar ligament when tension was applied. The patellar ligament was clamped between two 66-mm full rings (IMEX Veterinary) just distal to the patella; the rings were attached to the actuator so that vertical force could be applied to the patellar ligament[7] ([Fig. 4]). The complex was preloaded to 15 N of tensile force, and tension was applied at 1 mm/s until ultimate failure. Two orthogonal video cameras recorded each sample during testing, one facing cranial and one facing medial, which were used to confirm the cause of ultimate failure.

Fig. 4 Testing setup within the material testing machine. The potted tibias were positioned at 135 degrees from the patellar ligament. The ligament was clamped between two external fixator rings, which allowed the tensile force to be applied.

 

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Data Analysis

Nonparametric tests were chosen due to sample size. Load-displacement curves were generated from each tensile test. Ultimate failure force (N) and stiffness (N/mm) were determined from the load-displacement curves. Ultimate failure force (N) was the force recorded prior to a sudden reduction in force by ≥20%. Stiffness (N/mm) was calculated as the slope of the load-displacement between 20 and 80% of the displacement at ultimate failure. Ultimate failure force, stiffness, and tibial length were compared between groups using the independent samples median test. For pairwise comparisons, a Bonferroni correction was applied. The mode of failure was compared between groups using Fisher's exact test. The tests were designed to compare proportions of samples with patellar ligament failure versus other failure modes, and distal tibial crest failure versus other failure modes between the three experimental groups. Statistical testing was performed using Statistical Package for Social Sciences (IBM SPSS Statistics, Version 29, Chicago, Illinois, United States) and significance was set at p < 0.05.

 

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Results

The testing fixture failed for one sample in the Spacer Pin group; therefore, the results are reported for 9 samples in the Spacer Pin group and 10 in each of the TBW and 2 Pin groups. The overall median tibial lengths were 182 mm (range: 140–223 mm), and there were not any differences in median tibial length between groups (p = 0.645).

There were not any differences in ultimate failure force between groups (p = 0.301) including when performing pairwise comparisons (p = 1.000 for all comparisons): the TBW group had a median ultimate failure force of 1,175 N (range: 875–1,543 N), the 2 Pin group had an ultimate failure force of 1,041 N (range: 699–1,436 N), and the Spacer Pin group had an ultimate failure force of 1,222 N (range: 879–2,078 N; [Fig. 5A]).

Fig. 5 Box and whisker plots of ultimate failure force and stiffness for the tension band wire (TBW) group, 2 Pin group, and Spacer Pin group. No statistical differences were identified between groups for (A) ultimate failure force or (B) stiffness (p = 0.301 and 0.061, respectively) using the independent samples median test (SPSS; significance p < 0.05).

There were not any differences in stiffness between groups (p = 0.061) including when performing pairwise comparisons (p = 0.300–1.000): the TBW group had a median stiffness of 73 N/mm (range: 46–98 N/mm), the 2 Pin group had a stiffness of 74 N/mm (range: 35–91 N/mm), and the Spacer Pin group had a stiffness of 87 N/mm (range: 73–99 N/mm; [Fig. 5B]).

Patellar ligament failure was the most common mode of failure in all groups with no difference in proportion of ligament failure between groups (p = 0.620; [Table 1]). No TBW samples failed by distal tibial crest fracture, whereas in the 2 Pin and Spacer Pin groups, 4 and 3 samples, respectively, failed by distal tibial crest fracture, which was not significant (p = 0.210).

Table 1 Failure modes

Failure mode	TBW group	2 Pin group	Spacer Pin group[a]
Patellar ligament failure	n = 8	n = 6	n = 5
Distal tibial crest fracture/displacement	n = 0	n = 4[b]	n = 3
Diaphyseal tibial fracture	n = 1	n = 1[b]	n = 1
Tension band wire untwisting	n = 1	n/a	n/a
Breakage at pin tracts	n = 0	n = 0	n = 0

Abbreviations: TBW group, limbs that received complete osteotomy with a two-pin and tension band wire fixation; 2 Pin group, limbs that received partial osteotomy with a two-pin fixation; Spacer Pin group, limbs that received partial osteotomy with a spacer pin fixation.

^a One sample did not fail after withstanding 1821.3 N and experienced equipment failure.

^b One sample failed via both failure modes simultaneously.

 

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Discussion

This study investigated the mechanical viability of the spacer pin fixation for partial osteotomy TTT compared with two established techniques and found that the spacer pin technique could be an alternate stabilization method based on this acute load-to-failure model. We failed to reject the primary hypothesis: the mechanical properties of the Spacer Pin group were not different from the TBW or 2 Pin groups. The secondary hypothesis was partially rejected, as the most common mode of failure was patellar ligament rupture across groups.

In all groups, failure occurred at median tensile forces greater than 1,000 N, which is substantially larger than the approximate force thought to be exerted by the quadriceps during walking in medium to large dogs (∼240 N for a 25-kg dog).[20] This indicates that these procedures should all protect against failure acutely, at a single step, but it is not clear how each procedure would withstand cyclic testing. Additionally, the acute force exerted by the quadriceps during running or jumping may exceed the acute force at failure for any of the stabilization methods, underscoring the importance of activity restriction during the postoperative period.

Patellar ligament failure was the most common mode of failure across groups. Patellar ligament failure has been reported as a common mode of failure of TTT models secured by pin and TBW in cadaveric samples,[8] [10] but patellar ligament failure is not a common complication in vivo.[15] [16] Multiple explanations may account for this discrepancy. It is possible that the patellar ligament would withstand cyclic testing better than the bone or implants and, conversely, cyclic testing would highlight failure of the bone or implants. Second, only samples with well-performed procedures (e.g., osteotomy position) were tested, but imperfectly performed procedures may be a source of bone or implant failure in vivo. Finally, it is possible that cadaveric frozen and thawed patellar ligaments are weaker than in vivo ligaments or that our method of ligament clamping[7] was more traumatic to the ligament than other studies. However, because the ultimate failure force for the TBW group (1,175 N) was equal to or higher than that for TBW groups in similar prior studies (1,011 N[7] and 1,032 N[8]), it was determined that the method of ligament clamping was not inferior, and the bone and implants were still exposed to similar forces as prior studies.

Contrary to our hypothesis, no TBW or two-pin samples were noted to fail by fracture at the pin tract site, though this has been reported as a common site of failure in other acute force-to-failure cadaveric studies[8] and in clinical studies.[15] [16] [17] It is possible that our samples did not fail at the tuberosity pin tracts because the pins were placed within the footprint of the patellar ligament insertion rather than distal to it, eliminating the theoretical stress riser that could be generated by pin placement distal to the patellar ligament insertion; however, creation of a stress riser with more distal pin placement has not been proven.[15] Additionally, in clinical patients, pins may be removed/replaced to improve pin trajectory or change tuberosity position, which could weaken the tuberosity, but this was not performed in any tested samples in this study.

Between the two groups with pins placed through the tuberosity, we were unable to detect significant differences in mechanical properties between utilizing a TBW or leaving the distal tibial crest intact (“physiologic tension band”), suggesting that the robust intact distal tibial crest may oppose an acute pull of the patellar ligament similar to a TBW. Despite no difference in the failure force and stiffness of these constructs, placement of the TBW appeared protective against crest displacement as no samples with TBW failed by crest displacement, while four samples in the 2 Pin group failed by distal crest fracture and crest displacement. Prior biomechanical studies found that TBW placement had superior mechanical properties compared with pins alone, but these studies cannot be compared with the current study because a complete osteotomy was used for both groups.[7] [8] In a retrospective study following TTT using various stabilization methods, there was a positive association between leaving the distal tibial crest intact and a reduction in postoperative complication rates regardless of the stabilization technique used, but the proportion of distal crest remaining was not reported.[15] Based on this and the current study, it is possible that the use of the TBW and associated complications may be avoided by using a stabilization method without TBW if a robust distal tibial crest can be maintained.

There was also no difference in mechanical properties between the groups without a TBW: the 2 Pin and Spacer Pin groups. A proposed benefit of the spacer pin is the avoidance of pins placed through the tuberosity segment, theoretically reducing the risk of tuberosity segment fracture at the pin sites, though this was not demonstrated as there were not any tuberosity failures at the level of the pin tracts. Additional potential complications resulting from placing pins through the patellar ligament and sitting proud on or just proximal to the tuberosity include patellar tendinopathy, soft tissue irritation, seroma formation, or implant exposure. These soft tissue complications may be reduced or avoided by using an appropriately recessed spacer pin. Similar to the spacer pin technique, a retrospective clinical study described a technique utilizing a screw placed medial to the tuberosity segment.[12] This study reported a low postoperative complication rate (1.9% implant failure, 3% tibial tuberosity avulsion), supporting the potential for a reduced complication rate by avoiding placing implants through the tuberosity or the use of a TBW.[12] It should be noted that the exact position of the osteotomy in this study differs from that described by others, including the osteotomy described in the technique guide for the TTTT,[19] but was consistent across all groups.

Two different commercially available instruments were utilized in a modified fashion to standardize the surgical procedures. While both are designed to be secured to the proximal tibia using multiple pins, for the purpose of this study, the instruments were instead secured to jigs that the tibia was placed into to minimize confounding factors that could affect the mechanical strength of the bones. The osteotomy cutting guide was used because it has previously been reported to minimize variability in osteotomy position.[21] The TTTT was used to transpose the osteotomized tuberosity at a slow and consistent pace to bend the distal crest and avoid breakage. Distal crest fracture or failure to achieve transposition may have occurred if the TTTT had not been used due to the robust amount of intact distal crest. In clinical cases, an additional benefit of the TTTT is that patellar tracking can be checked prior to placing permanent implants, minimizing the number of defects created by placing multiple Kirschner wires or pins. The design of the TTTT has been modified since this study was performed, so that only one smaller diameter pin is placed in the proximal tibia to secure the instrument.

There are limitations associated with our study. The use of cadaveric samples cannot replicate the in vivo biomechanical environment postoperatively, including muscle forces, healing responses, or development of complications over time. Our study also utilized cadavers of varying sizes, leading to a wide range of mechanical results. To account for size differences, the specimens were assigned to groups using size matching to keep the average tibial length similar across groups to eliminate the effect of tibial size on the results of comparisons between groups. Additionally, the samples were obtained from medium to large breed dogs based on cadaver availability, but utilizing tibias from smaller breed dogs would have been more clinically relevant[1] and may have posed different challenges for the osteotomy and pin and wire placement that could have provided different results. Most importantly, a single force-to-failure test was utilized in this study, which does not account for cyclic loading/unloading during walking, running, jumping, and other activities that occur postoperatively during bone healing. A cadaveric cyclic testing study may better replicate the forces that can lead to failure in vivo, and a controlled prospective in vivo trial would be necessary to identify postoperative complications and to track time to bone healing to conclude that the spacer pin technique is clinically acceptable.

This study provides valuable insights into the biomechanical properties of three stabilization methods for TTT in MPL treatment. There were not any differences in ultimate failure force or stiffness between TBW, 2 Pin, and Spacer Pin groups under acute force-to-failure tensile testing. This study suggests that TBW use may not be necessary and that the use of a spacer pin may be equivalent to the use of two pins through the tuberosity when a robust distal crest is left intact. The spacer pin stabilization method shows promise as an alternative to the traditional methods of stabilization, but this fixation method should be further validated using ex vivo cyclic testing or in vivo studies.

 

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Conflict of Interest

M.P. created the Tibial Tuberosity Transposition Tool (TTTT) that was utilized in this study; however, this tool was not tested in this study but rather utilized for standardization between samples.

Acknowledgment

The authors thank OssAbility for the use of the tibial tuberosity cutting guide and Glenn Swan for assistance with creation of testing fixtures.

Authors' Contribution

A.S. and S.T. contributed to the study design, collection of data, data analysis, manuscript drafting, and editing. N.RO. contributed to the study design, collection of data, data analysis, and manuscript editing. B.P., P.N., and M.P. contributed to the study design and manuscript editing.

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Publication History

Received: 23 March 2024

Accepted: 04 October 2024

Article published online:
15 November 2024