Patellar fractures are a relatively uncommon occurrence in dogs and cats.1 In dogs, they can occur after direct trauma or indirect trauma due to intense quadriceps contraction or as a complication of tibial plateau-leveling osteotomies.2,3 The most common types of patellar fractures are transverse, comminuted, and apical fractures.4 Because strong distractive forces are generated by the quadriceps mechanism and the patella acts as the lever arm of the stifle extensor mechanism, the integrity of the patella is crucial for the proper function of the stifle joint.5,6 Internal fixation is indicated in simple 2-piece fractures when the fragments are of approximately equal size.7
The tension band wiring technique is commonly employed and indicated to convert distractive forces into compressive forces.8 However, there are several complications associated with this technique such as pin migration, skin irritation, and a high implant failure rate.9–11 Therefore, various novel fixation methods and materials have been introduced, including numerous modifications in tension band wiring methods, cerclage wiring, orthopedic sutures, cannulated screws with sutures, and plate and screw fixation in human medicine.12–14 Since animals have a smaller patella than humans, it is difficult or impossible to use many of the repair techniques applied in humans, such as cannulated screws.
Clinical comparison of steel wire with nonabsorbable high–molecular-weight polyethylene orthopedic sutures has been made in human transverse patellar fracture and has shown a lower reoperation rate for implant removal with comparable union rates.15–17 In a biomechanical study18 of feline patellar fracture repair, the combination of circumferential and figure-of-eight techniques with orthopedic sutures demonstrated sufficiently strong capacity. The ring pin is specially designed for tension band wiring and made of stainless steel, with a shape similar to a 1.5-mm Kirschner wire (K-wire) but includes an eyelet near the trailing end. In human medicine, modified tension band wiring techniques using ring pins have demonstrated improved clinical and radiographic outcomes for olecranon fractures.19 In addition, with orthopedic sutures, favorable outcomes and reduced complications in patellar fractures have been shown.20,21
To the best of our knowledge, no study has applied or evaluated ring pins in the canine fracture models to date. The aim of this study is to biomechanically evaluate the tension band wiring technique using the ring pin in canine cadaveric patellar fracture models by comparing 3 stabilization methods: a 1.5-mm K-wire with 0.8-mm metal wire for the K-wire and metal wire (KM) group, a 1.5-mm K-wire with No. 2 orthopedic suture for the K-wire and orthopedic suture (KS) group, and a 1.5-mm ring pin with No. 2 orthopedic suture for the ring pin and orthopedic suture (RS) group.
Methods
Specimen preparation
A total of 21 pelvic limbs from 11 adult dogs were collected and utilized, all of which had been euthanized or died for reasons unrelated to hind limb orthopedic pathologies. The mean weight of the canine cadavers was 13.7 ± 2.1 kg (11.3 to 17.4 kg), and a range of sizes suitable for applying implants were selected.7 The cadavers were frozen at −20 °C and then thawed at room temperature only when sampled and tested. After collection, the limbs promptly underwent the sampling and surgical processes and then were individually covered in saline-soaked gauze until being tested on the same day. Stifle joints, including the patellar tendon, quadriceps tendon, patellar groove, and tibial tuberosity were harvested, and all soft tissues were removed only leaving tendon and fascia (Figure 1). Mid patellar transverse osteotomies were performed with a 0.4-mm thickness handsaw. The limbs in each pair were selected at random and repaired with tension band wiring technique with one of the 3 implant systems: a 1.5-mm K-wire with 0.8-mm metal wire for the KM group, a 1.5-mm K-wire with No. 2 orthopedic suture for the KS group (FiberWire; Arthrex), or a 1.5-mm ring pin (Pin Tension Band System; Acumed) with No. 2 orthopedic suture for the RS group (Figure 2). There was no significant difference in body weight between the groups. A figure-of-eight pattern was applied to all the groups, and the wire or suture was tightened at a single point. The tension band wiring techniques and implant size were selected based on previous studies.19,22
Photographs of the specimen after the preparation procedure and while biomechanical testing. A total of 21 pelvic limbs were used from 11 canine adult cadavers. A—The patella and associated ligaments, tendons, and bony structures such as patellar groove and tibial tuberosity were harvested. The patella was osteotomized transversely. B—The tensile force was applied to the specimen using the universal testing machine, while the quadriceps tendon and tibia were fixed.
Citation: American Journal of Veterinary Research 2025; 10.2460/ajvr.24.12.0378
Illustrations and photographs of 3 fixation methods for transverse patellar fracture models in this study. All methods were the same tension band wire techniques using a figure-of-eight pattern with a single pin, but the materials were different. A and D—The Kirshner wire (K-wire) and metal wire (KM) group (n = 7) was stabilized by a 1.5-mm K-wire and metal wire. B and E—The K-wire and orthopedic suture (KS) group was stabilized by a 1.5-mm K-wire and orthopedic suture. C and E—The ring pin and orthopedic suture (RS) group was stabilized by a 1.5-mm RS.
Citation: American Journal of Veterinary Research 2025; 10.2460/ajvr.24.12.0378
The KM group
A 1.5-mm K-wire and 0.8-mm metal wire were applied (Figure 2). A 1.2-mm drill bit was used to create a centrally located drill hole from the cut surface of the proximal patella to the proximal pole (retrograde) to ensure appropriate pin placement and then passed from the proximal to the distal pole using the drill guide. A 1.5-mm diameter K-wire was placed longitudinally from the drill hole in the distal bone fragment, the fracture was then reduced, and the pin was passed into the proximal fragment, exiting at the proximal pole of the patella. The K-wire protruding from both sides was cut, leaving 3 mm on each side. A figure-of-eight wiring with 0.8-mm diameter orthopedic wire was applied to the pin, passing behind the proximal and distal ends of the K-wire. The wire was tightened at a single point with 3 to 4 twists, and then the ends were bent and cut.
The KS group
A 1.5-mm K-wire and No. 2 orthopedic suture were applied (Figure 2). A pilot hole was drilled with a 1.2-mm diameter drill bit and a single 1.5-mm K-wire was drilled in the same manner as in the KM group. A figure-of-eight wiring with double strands of orthopedic suture was applied, passing behind the proximal and distal ends of the K-wire. After a figure eight was formed, the suture was tightened manually by a Nice knot, which is a double-stranded knot passing both strands at one end through a loop on the other side, followed by 3 half hitches and 2 square knots. The suture method and implant size were selected based on previous studies.17,23,24
The RS group
A 1.5-mm ring pin and No. 2 orthopedic suture were applied (Figure 2 and Figure 3). A pilot hole was drilled in the same manner as in the other groups. Then, a 1.5-mm ring pin was passed from the distal bone fragment, the fracture was then reduced, and the pin was passed into the proximal fragment, exiting at the proximal pole of the patella. Therefore, the pin head was placed at the distal pole of the patella. The suture method was the same as in the KS group, but the suture was passed through the eyelet hole of the pin at the distal pole of the patella instead of passing behind. Before the suture was completely tightened, the eyelet head was seated into the ligaments using a snapper and mallet to engage with the bony surface of the distal fragment. After the suture was tightened, the tail was broken from the pins with the snapper, and the trocar end was also cut, leaving 3 mm.
Illustrations and photographs explaining surgical procedures for the application of the ring pin in patellar fracture models. A and F—Following predrilling procedures, a ring pin passed from the distal bone fragment. B and G—Double strands of No. 2 orthopedic suture were passed through the eyelet hole and presutured. C and I—The pin head was seated using a snapper and mallet. H—Instruments used for the seating procedure (eyelet pin, snapper, and mallet). D and J—Suture was fully tightened. E and K—The tail was broken with the snapper, and the proximal end was also cut.
Citation: American Journal of Veterinary Research 2025; 10.2460/ajvr.24.12.0378
Biomechanical evaluation
The limbs were positioned on a wooden plate at a normal standing angle of 135°. The wooden plate was mounted on the lower tensile grip of the universal testing machine (Instron 5585; Instron Corp), while the quadriceps tendon, wrapped in gauze to prevent slippage, was held by the upper grip (Figure 1). Care was taken to keep the patella in contact with the trochlea throughout the test in order not to interfere with the tension band mechanism. Then, a tensile force with a crosshead speed of 10 mm/min was applied. Meanwhile, displacements of 1, 2, and 3 mm for the patella fracture were measured on the lateral side during the test. Failure was defined as the point at which the load showed a sudden drop or the implant broke or bent. Failure types were specified and raw data were extracted to determine the maximum failure load of each sample.
Statistical analysis
The SPSS program (Statistical Program for Social Sciences, version 30.0.0; IBM Corp) was utilized for statistical analysis. As all of the groups had less than 10 samples, the Kruskal-Wallis test was applied to find the statistical significance between the groups. When the P value was < .05, it was defined to be significant. If there was significance among the groups, the significance between each of the groups was sought with the Mann-Whitney test with Bonferroni correction. A P value of < .017 was considered significant.
Results
Load at 1-, 2-, and 3-mm displacement
The median load and IQR at 1-mm displacement were 95.1 ± 53.5 N for the KM group, 161.3 ± 101.7 N for the KS group, and 141.1 ± 142.4 N for the RS group (Table 1). At 1-mm displacement, there were no significant differences among the 3 groups (P > .05).
Loads at 1-, 2-, and 3-mm displacement and maximum failure point of repaired patellar fracture models under tensile force during the biomechanical testing.
| Load (N) | KM group | KS group | RS group |
|---|---|---|---|
| Displacement of 1 mm | 95.1 ± 53.5a | 161.3 ± 101.7a | 141.1 ± 142.4a |
| Displacement of 2 mm | 150.6 ± 46.5a | 209.7 ± 143.9a,b | 244.1 ± 95.3b |
| Displacement of 3 mm | 200.1 ± 62.8a | 247.8 ± 116.9a,b | 341.0 ± 149.1b |
| Maximum failure point | 270.3 ± 74.2a | 298.7 ± 138.7a | 421.9 ± 51.5b |
Values reported are median ± IQR. The Kirshner wire (K-wire) and metal wire (KM) group was stabilized by a 1.5-mm K-wire and metal wire. The K-wire and orthopedic suture (KS) group was stabilized by a 1.5-mm K-wire and orthopedic suture. The ring pin and orthopedic suture (RS) group was stabilized by a 1.5-mm RS.
Values with different letters are significantly (P < .017) different.
The median load and IQR at 2-mm displacement were 150.6 ± 46.5 N for the KM group, 209.7 ± 143.9 N for the KS group, and 244.1 ± 95.3 N for the RS group (Table 1). At 2-mm displacement, a significant difference was observed between the KM group and RS group (P = .007 < .017), whereas the KM group and KS group (P > .017) and the KS group and RS group (P > .017) were not statistically different.
The median load and IQR at 3-mm displacement were 200.1 ± 62.8 N for the KM group, 247.8 ± 116.9 N for the KS group, and 341.0 ± 149.1 N for the RS group (Table 1). At 3-mm displacement, a significant difference was observed between the KM group and RS group (P = .002 < .017), whereas the KM group and KS group (P > .017) and the KS group and RS group (P > .017) were not statistically different.
Maximum failure loads
At maximum failure load, the median load and IQR were 270.3 ± 74.2 N for the KM group, 298.7 ± 138.7 N for the KS group, and 421.9 ± 51.5 N for the RS group (Table 1). Significant differences were observed between the KM group and RS group (P = .001 < .017), as well as the KS group and RS group (P = .011 < .017), whereas the KM group and KS group (P > .017) were not statistically different (Figure 4).
Box-and-whisker plots of maximum failure loads of repaired patellar fracture models under tensile forces. The results from the biomechanical testing of the specimens described in Figures 1 and 2 were used. The solid line within the box represents the median, the lower and upper limits of the box represent the OR, and the whiskers delimit the range. Groups labeled with different letters are statistically significantly different from each other (P < .017). The Kirshner wire (K-wire) and metal wire (KM) group was stabilized by a 1.5-mm K-wire and metal wire. The K-wire and orthopedic suture (KS) group was stabilized by a 1.5-mm K-wire and orthopedic suture. The ring pin and orthopedic suture (RS) group was stabilized by a 1.5-mm RS.
Citation: American Journal of Veterinary Research 2025; 10.2460/ajvr.24.12.0378
Types of failure
All specimens failed either due to pin cut through of the bone fragment or pin pullout from the bone fragment with or without wire elongation or bending (Figure 5). In the KM group, the most frequent failure type was pin pullout (n = 4), followed by pin cut through (3). All the specimens in the KM group showed wire elongation and one specimen showed pin bending. Pin cut through was the most common type in both the KS group and RS group. Pin bending was observed in one specimen of the KS group and was not observed in the RS group. No failure was caused by ligament rupture.
Photographs of the failure modes under tensile force in the repaired patellar fracture models. The methods applied to the Kirshner wire (K-wire) and metal wire (KM) group, K-wire and orthopedic suture (KS) group, and ring pin and orthopedic suture (RS) group are described in Figure 2. A—Pin pull-through from the bone fragment and pin bending were observed with wire elongation in the KM group. B and C—Pin cut through of bone fragments was observed in the KS group and RS group.
Citation: American Journal of Veterinary Research 2025; 10.2460/ajvr.24.12.0378
Discussion
As noted in previous studies,20 the ring pin allows the wire to be hooked onto the head hole, reducing migration and providing self-locking while minimizing soft tissue irritation. Since the eyelet hole is only on one side, the pin provides resistance to tensile forces primarily through the wire rather than through its own compression, similar to K-wires. However, the maximum failure loads measured in the ring pin group showed significantly greater resistance compared to the other two groups. This may be attributed to the head seating procedure, which provides enhanced stability.4 This procedure not only allows the wire and pin to be positioned closer to the bone but also shortens the length of the pin and wire compared to other methods because the wire is tightened and the pin is trimmed after the seating process. In patellar fractures, the force applied by the quadriceps muscle primarily exerts rotational or bending forces on the implant.
Shortening the implant length reduces the torque applied to the implant, suggesting that the ring pin’s shorter pin-wire length may be biomechanically advantageous compared to the K-wire tension band wiring method. This makes the ring pin a promising implant not only for patellar fractures but also for other fractures in dogs that follow a similar mechanism.
In the 1-, 2-, and 3-mm displacement measurements, the RS group showed significantly greater strength to displacement at 2 and 3 mm compared to the KM group (K-wire and metal wire), with no significant differences observed between other groups and 1-mm displacement. While human studies suggest that a displacement of 2 to 3 mm or a shift of 1 to 4 mm indicates the need for surgical correction, there is no such guideline for dogs.16 Joint reaction forces applied on the knee during gait are estimated to be about 2 to 3 times body weight in a biomechanical study of human medicine.25 In dogs, the maximal vertical force on the hind limbs when standing is approximately 20% to 30% of body weight, and the peak vertical force during trotting ranges from 65% to 107% depending on breed.6,26 However, detailed studies on tensile forces acting on stifle structures under weight bearing and the threshold of displacement that necessitates surgical intervention in dogs are lacking. If we assume the implant must withstand the at least maximum vertical force applied under weight bearing, then based on the weight of this study’s sample, it should resist 40.3 N while walking (30% of body weight) and 143.8 N while trotting (107% of body weight). Under this criterion, none of the samples would show displacement during walking, the KM group can exhibit a 1-mm displacement during trotting based on our data (Table 1). Further studies on the forces acting on the canine stifle joint and the displacement that warrants surgical intervention in patellar fractures are needed.
In the KM group, all samples showed wire elongation, and more samples failed due to pin pullout than pin cut through of the bone (Figure 5). In contrast, the KS group and RS group showed more instances where the pin cut through the bone fragment. While these results are consistent with previous studies indicating high-molecular orthopedic sutures retain their stiffness longer, further studies with larger samples are needed to confirm with statistical significance. The RS group displayed no pullout from the distal bone fragment and no pin bending. The absence of pin bending observed in this group can be attributed to the reduced torque acting on the implant, as previously mentioned.
Although various tension band wiring techniques have been introduced, this study employed a single K-wire with a single-point twist. In human studies,27 various methods of figure-of-eight patterns (the number, location, and orientation) can affect biomechanical strengths, suggesting that different results may be observed in canine samples with different techniques. In this study, the ring pin was inserted with the head positioned distally, opposite to the proximal head orientation in humans. This decision was made by considering anatomical differences in patellar shape and the occurrence of skin or patellar ligament irritation in dogs; however, further research is necessary to determine which orientation provides better surgical outcomes in vivo.
This study has several limitations. First, the small sample size may reduce the statistical power of the findings, highlighting the need for studies with larger samples for more reliable and detailed results. Second, biomechanical evaluations were limited to only tensile force tests at fixed angles, which may not fully replicate in vivo motion. Experiments across various knee angles and repeated cyclic load tests are needed for more complete results. Third, the use of cadavers may mean that ligament or cartilage tissue strength does not match that of living tissue, and variations in sample conditions may have affected the results.
Nevertheless, to the best of our knowledge, there is no biomechanical comparison study to date on repair methods using the ring pin or the ring pin with orthopedic sutures in fractures in dogs, including the patella. This study indicates that the RS combination withstands greater force compared to conventional methods in the canine transverse patellar fracture models and has the potential to reduce complication rates. This surgical technique could also be considered a viable option for other fractures in dogs, particularly those requiring resistance to tensile forces.
Acknowledgments
None reported.
Disclosures
The authors have nothing to disclose. No AI-assisted technologies were used in the composition of this manuscript.
Funding
The authors have nothing to disclose.
References
- 1.↑
Gwinner C, Märdian S, Schwabe P, Schaser KD, Krapohl BD. Current concepts review: fractures of the patella. MS Interdiscip Plast Reconstr Surg DGPW. 2016;5:Doc01.
- 2.↑
Rutherford S, Bell JC, Ness MG. Fracture of the patella after TPLO in 6 dogs. Vet Surg. 2012;41(7):869–875. doi:10.1111/j.1532-950X.2012.01018.x
- 3.↑
Langley-Hobbs SJ. BSAVA Manual of Canine and Feline Fracture Repair and Management. 2nd ed. British Small Animal Veterinary Association; 2016.
- 4.↑
Cruciani B, Gagnon D, Freire M, Montasell X. Excellent long-term clinical outcome in a young athletic dog following surgical complications of a comminuted patellar fracture and patellar ligament rupture. Can Vet J. 2021;62(9):945–950.
- 5.↑
Vasseur PB. Stifle joint. In: Slatter D, ed. Textbook of Small Animal Surgery. 3rd ed. Saunders; 2003:2126–2129.
- 6.↑
Gibert S, Kowaleski M, Matthys R, Nützi R, Serck B, Boudrieau R. Biomechanical comparison of pin and tension-band wire fixation with a prototype locking plate fixation in a transverse canine patellar fracture model. Vet Comp Orthop Traumatol. 2016;29(1):20–28. doi:10.3415/VCOT-15-07-0115
- 7.↑
DeCamp CE, Johnston SA, Déjardin LM, Schaefer SL, eds. Brinker, Piermattei, and Flo’s Handbook of Small Animal Orthopaedic and Fracture Repair. 5th ed. Elsevier; 2016:590–593.
- 8.↑
Kowaleski MP, Boudrieau RJ, Pozzi A. Stifle joint. In: Veterinary Surgery: Small Animal: 2-volume set. Elsevier Health Sciences; 2013:1162–1164.
- 9.↑
Kumar G, Mereddy PK, Hakkalamani S, Donnachie NJ. Implant removal following surgical stabilization of patella fracture. Orthopedics. 2010;33(5):301–304. doi:10.3928/01477447-20100329-14
- 10.
Melvin SJ, Mehta S. Patellar fractures in adults. J Am Acad Orthop Surg. 2011;19(4):198–207. doi:10.5435/00124635-201104000-00004
- 11.↑
LeBrun CT, Langford JR, Sagi HC. Functional outcomes after operatively treated patella fractures. J Orthop Trauma. 2012;26(7):422–426. doi:10.1097/BOT.0b013e318228c1a1
- 12.↑
Schuett DJ, Hake ME, Mauffrey C, Hammerberg EM, Stahel PF, Hak DJ. Current treatment strategies for patella fractures. Orthopedics. 2015;38(6):377–384. doi:10.3928/01477447-20150603-05
- 13.
Steinmetz S, Brgger A, Chauveau J, Chevalley F, Borens O, Thein E. Practical guidelines for the treatment of patellar fractures in adults. Swiss Med Wkly. 2020;150:w20165. doi:10.4414/smw.2020.20165
- 14.↑
Liu J, Ge Y, Zhang G, et al. Clinical outcomes of cannulated screws versus ring pin versus K-wire with tension band fixation techniques in the treatment of transverse patellar fractures: a case-control study with minimum 2-year follow-up. Biomed Res Int. 2022;2022:1–10. doi:10.1155/2022/5610627
- 15.↑
Patel VR, Parks BG, Wang Y, Ebert FR, Jinnah RH. Fixation of patella fractures with braided polyester suture: a biomechanical study. Injury. 2000;31(1):1–6. doi:10.1016/S0020-1383(99)00190-4
- 16.↑
Gosal HS, Singh P, Field RE. Clinical experience of patellar fracture fixation using metal wire or non-absorbable polyester—a study of 37 cases. Injury. 2001;32(2):129–135. doi:10.1016/S0020-1383(00)00170-4
- 17.↑
Wright PB, Kosmopoulos V, Coté RE, Tayag TJ, Nana AD. FiberWire® is superior in strength to stainless steel wire for tension band fixation of transverse patellar fractures. Injury. 2009;40(11):1200–1203. doi:10.1016/j.injury.2009.04.011
- 18.↑
Lee MY, Ahn TH, Son SI, Kim HY. Biomechanical evaluation of three fixation methods in feline transverse patellar fracture model. J Feline Med Surg. 2023;25(5):1098612X231172630.
- 19.↑
Okamoto M, Namba J, Kuriyama K, Miyamura S, Yokoi H, Yamamoto K. Surgical technique in tension band wiring method for selected comminuted olecranon fractures. Eur J Orthop Surg Traumatol. 2020;30(2):237–242. doi:10.1007/s00590-019-02551-y
- 20.↑
Yotsumoto T, Nishikawa U, Ryoke K, Nozaki K, Uchio Y. Tension band fixation for treatment of patellar fracture: novel technique using a braided polyblend sutures and ring pins. Injury. 2009;40(7):713–717. doi:10.1016/j.injury.2008.10.032
- 21.↑
Zhu X, Huang T, Zhu H, et al. A retrospective cohort study on prevalence of postoperative complications in comminuted patellar fractures: comparisons among stabilized with cannulated-screw, Kirschner-Wire, or ring-pin tension bands. BMC Musculoskelet Disord. 2021;22(1):60. doi:10.1186/s12891-020-03936-5
- 22.↑
Fox DB. Repair of patellar fractures. In: Bojrab MJ, Waldron DR, Toombs JP, eds. Current Techniques in Small Animal Surgery. 5th ed. Saunders; 2014:1061–1063.
- 23.↑
Meyer DC, Bachmann E, Lädermann A, Lajtai G, Jentzsch T. The best knot and suture configurations for high-strength suture material. An in vitro biomechanical study. Orthop Traumatol Surg Res. 2018;104(8):1277–1282. doi:10.1016/j.otsr.2018.08.010
- 24.↑
Hwang BM, Kim MS. Biomechanical comparison of metal wire and FiberWire as tension band techniques: an ex vivo study. Iran J Vet Res. 2021;22(2). doi:10.22099/ijvr.2021.37914.5523
- 25.↑
Kim HJ, Fernandez JW, Akbarshahi M, Walter JP, Fregly BJ, Pandy MG. Evaluation of predicted knee-joint muscle forces during gait using an instrumented knee implant. J Orthop Res. 2009;27(10):1326–1331. doi:10.1002/jor.20876
- 26.↑
Budsberg SC, Rytz U, Johnston SA. Effects of acceleration on ground reaction forces collected in healthy dogs at a trot. Vet Comp Orthop Traumatol. 1998;11(01):15–19. doi:10.1055/s-0038-1632610
- 27.↑
Ali M, Kuiper J, John J. Biomechanical analysis of tension band wiring (TBW) of transverse fractures of patella. Chin J Traumatol. 2016;19(5):255–258. doi:10.1016/j.cjtee.2016.06.009
