Patellar luxation is one of the most common hindlimb orthopedic diseases in dogs (1–5). Patellar luxation can be medial, lateral, or bidirectional, with the vast majority being medial (1, 2, 6, 7). Small breed dogs are 12 times more likely to suffer from the disease and are affected bilaterally in 52 to 65% of cases (7). The pathogenesis of congenital patellar luxation is still unclear, but malalignment of the quadriceps mechanism is considered a major predisposing factor (8–10).

Surgical treatment is recommended for the symptomatic patient (6, 7). Tibial tuberosity transposition (TTT) is performed to realign the quadriceps mechanism (6, 9, 11–13) and reduce the risk of reluxation (2, 14, 15). Several techniques have been suggested for the fixation of the TTT, including 2 pins and tension band wire (2Pins + TBW) or 2 pins (2Pins). Previous studies suggested that the pins should be placed approximately perpendicular to the long axis of the tibia in order to best counteract the tensile force of the patellar tendon (15–19).

The reported implant-associated complication rate of TTT for MPL varies from 18 to 43%, with a high rate of major complications (14, 15, 17, 20–23). Specifically, complications can include implant breakage, implant migration, and avulsion of the tibial tuberosity, as well as infection (15, 24). Most of the complications are associated with the smooth pins placed from cranial to caudal to stabilize the tuberosity fragment to the main tibial segment, which can cause soft tissue irritation or can migrate (14, 15, 17, 20, 23, 25, 26). A fixation technique of the tibial tuberosity that does not require pins might be beneficial to reduce the rate of complications of TTT.

A TTT technique with a single-length orthopedic wire has been proposed to avoid complications associated with the use of pins (27). The proposed technique utilizes a single-length orthopedic wire placed as a modified hemicerclage (MH) in a tension band fashion, without the use of pins through the tuberosity. Similar studies employing quasi-static load-to-failure tests have been conducted, focusing on different fixation methods, however predominantly involving larger breed subjects (12, 18, 28–30). This biomechanical study explicitly focuses on the initial validation of the novel technique’s biomechanical properties compared to well-established techniques and aims to primarily assess the mechanical strength of the fixation method.

The objectives of the study are (1) to determine the stiffness, load to failure, and mode of failure of a novel fixation method for TTT with a single-length orthopedic wire placed as a modified hemicerclage in tension band fashion (MH) in a small dog model, and (2) to compare these biomechanical testing results of the novel fixation to other tibial tuberosity fixation techniques (2Pins and 2Pins + TBW). We hypothesized that (1) there would be no difference in stiffness and load to failure between the novel fixation technique and the 2Pins + TBW and that (2) the novel fixation technique would fail at a higher load and be stiffer compared to the 2Pins fixation method.

Thirty tibias (n = 30) from skeletally mature small-breed dogs, euthanatized for reasons unrelated to this study, were collected for the study. All the cadaveric specimens were donated for research purposes by their owners, and written consent was obtained according to the institute and national regulations. Tibias from chondrodystrophic dogs or dogs with any stifle pathology were excluded from the study. Specimens underwent radiographic evaluation prior to testing to confirm skeletal maturity and to exclude any stifle pathologies. Immediately after euthanasia, all tibial specimens were dissected free from the soft tissues except the patellar tendon leaving the patella attached and wrapped in saline-soaked towels, then stored at −20° C for subsequent use. Prior to testing, the tibial specimens were thawed at room temperature for 24 h.

All cadaveric specimens in this study were skeletally mature dogs, including nonchondrodystrophic mixed breeds (n = 25), Poodle (n = 3), and Fox Terrier (n = 2), with median weight 7.7 kg (range = 5.2 kg – 13.1 kg) (mean weight = 7.4 kg).

In this study we found that a novel TTT fixation technique with a single orthopedic wire placed as a tension modified hemicerclage was not different from a 2Pins + TBW technique in stiffness and load at failure. Both of these techniques were stronger than the 2Pins technique. The forces recorded at complete failure in the 3 treatment groups were within the normal range of the quadriceps muscle forces, which was estimated to be approximately 94.8% of a patient’s body weight during walking (31). This result suggests that all three techniques may be strong enough to withstand normal forces on the patellar ligament during walking. However, mechanical failure data collected in a cadaveric study are difficult to interpret clinically. Further studies are warranted to determine the safety and efficacy of the novel technique in clinical cases.

The load to failure reported in our study was approximately two times higher than that reported in a similar study performed with tibiae from raccoon dogs, which have similar mean body weights to the specimens used in the present study (12). The raccoon dog study reported mean weights of 7.1 kg and 6.2 kg. In comparison, our study presented a mean weight of 7.4 kg for the specimens. The mean stiffness, similarly, was higher in the constructs in the current study. A notable difference was observed in the mode of failure: our study had fewer instances of tibial fractures compared to the raccoon dog study. The occurrence of bone fractures at lower forces in the raccoon dog study may be attributed to several factors. These include inherent species-based differences in bone quality, variations in the age of the specimens, and the methodologies used for specimen storage and preparation, particularly freeze/thaw processes (29, 32, 33). Such factors can significantly impact the biomechanical properties of the bones, thereby influencing the outcomes of biomechanical testing.

Our results confirmed that 2 pins combined with a tension band counteract the tensile force applied by the patellar ligament and therefore withstands greater forces than 2 pins alone. Similarly, based on our results, we theorize that the MH counteract tensile forces similarly to TBW, because the wire loops both in a craniocaudal and mediolateral direction, mimicking the direction of a pin and a TBW (Figure 2). Thus, this construct offers similar biomechanical advantages to TBW without the need for pins that may migrate or cause morbidity (14, 15, 20).

While the implant size would be modified on a case-by-case basis in clinical cases, (7, 34) we elected to use standardized implant sizes, including 1.0 mm 316LVM pins and/or 0.6 mm stainless steel orthopedic wire, across all specimens. While 1.0 mm diameter pins are small, the presence of fragment fractures as a mode of failure in the current study may indicate that two 1.0 mm pins horizontally aligned might have been oversized for some of the specimens. Alternatively, smaller pins or vertical alignment of the pins may be preferable in small breed dogs in hopes of lessening the incidence of tibial tuberosity fracture if a construct utilizing pins through the tuberosity is going to be used (18).

The observed modes of failure in our study were notably distinct for each fixation group, shedding light on the biomechanical behaviors of different techniques. The single orthopedic wire group exhibited failure primarily at the site where the wire intersects the tuberosity, highlighting potential concerns about the structural integrity of the tuberosity under load. In contrast, the 2Pin group’s primary mode of failure was pin pullout, suggesting that this method may not provide sufficient anchorage. The 2Pin + TBW group showed failure through both wire breakage and pin pullout, indicating a combination of material and anchorage weaknesses. This variance in failure modes is intriguing when compared to other studies. For instance, in a study evaluating pins and TBW, the primary failure mode involved the untwisting of the wire and bending of the pins, rather than breakage or pullout (28). Similarly, another study focusing on 2Pins and 2Pins + TBW constructs reported patellar ligament failure as the main issue (18) which was not observed in our study at all. Possible reasons for differences in the mode of failure are the smaller specimen size, lower stiffness, inherent variability in conformation, potential differences in specimen handling such as freeze–thaw cycles, differences in surgical technique such as pin orientation or wire twisting, and differences in bone quality between specimens. These potential factors underline the complexity of biomechanical testing and the challenge of directly comparing different studies. It is crucial to consider these variables when interpreting our results and those of similar research.

The results of this ex vivo cadaveric study should be interpreted considering some limitations. The unidirectional tensile load-to-failure mechanical testing does not replicate physiologic loading or cyclic fatigue encountered in the postoperative convalescent period. Therefore, translation of the results to a clinical setting should be done with caution. Another limitation is the broad range of body weight of the dogs used for the study. We selected small breed dogs because of the clinical significance, which may be considered more relevant in comparison to other similar studies that utilized larger cadavers (12, 18, 28–30). The wide range of tibia size would likely have caused variability in mean stiffness and maximum load at failure, as well as failure modes. Our results show that the proposed technique can withstand tensile loads similar to a tension band wire construct. However, there are uncertainties regarding the behavior of the wire within the crest fragment and its interaction with trabecular bone density, potentially affecting the final position of the fragment. Future studies, such as CT imaging, will be necessary to measure the degree of lateral displacement. Nonetheless, our study focuses on defining the biomechanical properties of the construct. Despite these limitations, the results from the present study are comparable to previously reported studies, when differences in weight categories are taken into account (12, 18, 28–30). A direct comparison to other studies should be made with caution due to the differences in specimen weight range, the small sample size, breed/species, and, consequently, implant size.

In framing the results of our study, it’s essential to emphasize that this investigation represents an initial biomechanical assessment, primarily focused on establishing the basic mechanical viability of the novel TTT fixation technique using a single orthopedic wire. This approach aligns with the foundational step in introducing any new surgical method, where the primary concern is to establish that the technique is mechanically sound and can at least withstand normal physiological loads. By demonstrating comparable stiffness and load at failure with the 2Pins + TBW method, and superior performance compared to the 2Pins method, our study suggests that the novel technique is mechanically stable enough for further consideration. For the sake of consistency and comparability with existing biomechanical literature, we conducted our tests with the stifle positioned at 135° extension, as established in previous studies (12, 18, 28, 30). A consideration when applying the modified hemicerclage technique is the potential for soft tissue trauma, primarily because the tibialis cranialis muscle must be carefully prepared to facilitate the passage and retrieval of the wire. However, based on our clinical experience, the extent of trauma incurred is generally acceptable and can be effectively managed within the surgical process.

This study reported a novel fixation technique for TTT that utilizes a single-length orthopedic wire placed as a modified hemicerclage in a tension band fashion. The stiffness and load at failure of the constructs stabilized using this new technique were not different from the constructs stabilized with 2Pins + TBW. These results suggest that this new technique may have potential benefits, such as reducing implant-associated complications. It is important to underline that while our study contributes valuable initial data regarding the mechanical stability of the new fixation method, it cannot be considered conclusive regarding the technique’s clinical applicability. The results point toward a need for further research, particularly involving clinical trials. Such studies would not only provide more comprehensive data on the method’s mechanical resilience over time but also offer insights into potential advantages regarding reduced necessity for implant removal, a key consideration given the complications associated with traditional pin-based methods.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

Ethical approval was not required for the studies involving animals in accordance with the local legislation and institutional requirements because Tibias from skeletally mature dogs, euthanatized for reasons unrelated to this study, were collected for the study. All the cadaveric specimens were donated for research purposes by their owners, and written consent was obtained according to the institute and national regulations. Written informed consent was obtained from the owners for the participation of their animals in this study.

PN: Writing – review & editing, Writing – original draft, Visualization, Validation, Project administration, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. RC: Writing – review & editing, Writing – original draft, Visualization, Project administration, Investigation, Formal analysis, Data curation. NK: Methodology, Writing – review & editing, Writing – original draft, Visualization, Validation, Conceptualization. AP: Writing – review & editing, Writing – original draft, Visualization, Validation, Supervision, Resources, Methodology. ST: Validation, Supervision, Methodology, Writing – review & editing, Writing – original draft, Visualization. BP: Writing – review & editing, Writing – original draft, Visualization, Validation, Supervision, Software, Resources, Project administration, Methodology, Investigation, Formal analysis, Data curation, Conceptualization.