Use of a Novel External Skeletal Fixation System (Exvet) for Long Bone Fracture Stabilization in Young Cats

Abstract

Objective

To describe the use of a novel external skeletal fixation (ESF) system for long-bone fracture stabilization in 30 cats.

Study Design

Medical records of consecutive cases with femoral, tibial, humeral, and antebrachial fractures in cats, repaired with the Exvet ESF system at two different academic institutions between September 2022 and December 2023, were included. Data were collected regarding signalment, surgical approach, weight of Exvet frame, postoperative complications/additional treatments, days to dynamization, days to ESF removal, radiographic evaluation, and clinical outcome.

Results

Thirty cases met the inclusion criteria. The mean age was 23.5 months (range: 2 months to 13 years), and the mean body weight was 3.7 kg (range: 750 g to 7.35 kg). Fracture distribution was 56.6% tibial, 23.3% femoral, 13.3% antebrachial, and 6.66% humeral. Reduction was achieved in 63.3% of fractures in a closed fashion, 23.3% were reduced by a limited open approach, and 13.3% were fixed in an open fashion. The median weight of the ESF frames was 27.8 g (range: 13–42.5). The mean time to ESF removal was 32 days (range: 16–63 days). Full functional outcome was achieved in 79.9% of cases, 16.6% had an acceptable outcome immediately after ESF removal, and in 3.3% the outcome was unfavorable.

Conclusion

Our findings show that the novel Exvet ESF system seems a safe and effective option for mini-invasive treatment of long-bone fractures in young cats.

Keywords

circular ESF - fracture fixation - external skeletal fixation - orthopaedic implants - orthopaedic surgery

Introduction

Long-bone fractures are relatively common in cats due to high-energy trauma such as road traffic accidents or falls from a height (high-rise syndrome).[1] [2] Incidences of long-bone fractures in cats have been reported: one study including 22,258 cats, found that femoral fractures were the most common (36.5%), followed by tibial fractures (19%), antebrachial (16%), and humeral fractures (8%).[2] [3] [4] [5]

Several treatment options are available for long-bone fracture repair in cats, such as bone plates,[6] [7] [8] interlocking nails,[9] [10] intramedullary pins and wires,[11] plate-rod construct,[12] and external skeletal fixation (ESF).[6] [13] All procedures aim to achieve bone healing as fast as possible and early ambulation while preserving the neurovascular structures.[1] [13] Effective fracture treatment requires adequate mechanical stability and preservation of soft tissues. These two principles must be balanced for optimal bone healing. This concept is referred to as the “balance in fracture treatment.”[14] ESF systems are quite suitable for this concept due to their advantages, such as closed fashion application.[14] [15] [16]

ESF has been commonly used for traumatic, developmental, and degenerative abnormalities in veterinary orthopaedics.[4] [15] [17] [18] [19] Several ESF systems have been developed such as FESSA,[4] OMU-Dogfix,[13] and IMEX SK system[20] [21] to improve the original KE design,[15] and ESFs, including the Ilizarov system.[22]

One of these advanced ESF systems is Exvet (Exvet^®, Ankara, Türkiye) fixators developed in Tübitak (Scientific and Technological Research Council of Türkiye, 1512 Individual Young Initiative Program). The Exvet ESF system consists of an updated version of aluminum rings, arches, carbon fiber rods, and other devices.

Exvet fixators are designed for more practical ESF applications with less components. The design of Exvet frames avoids some components such as hexagonal nuts, bolts, and threaded stainless steel rods. Furthermore, carbon fiber rods can be used as connecting rods between rings and arches. Therefore, linear and circular systems are not considered as two separate products and can be combined into one single system. These features are the major differences of this new system.

The aim of this retrospective study was to describe the use of Exvet fixators and evaluate fracture healing in the treatment of long-bone fractures in 30 cats.

Materials and Methods

Inclusion Criteria

The medical records of all cases with long-bone fractures treated using Exvet fixators at Ankara University and Ondokuz Mayıs University Veterinary Hospital between September 2022 and December 2023 were reviewed. All records of cats with long-bone fractures treated with Exvet fixators without other orthopaedic conditions and free from systemic diseases were included. Cases with incomplete medical records such as missing radiographic data were excluded. Medical records with final evaluation of the data after ESF removal were considered as completed.

The data of the study were obtained with the medical records of the cases including signalment, history, location of the fracture, surgical approach, fixator configuration, weight of the frame, surgery time, postoperative complications/additional treatments, concomitant injuries, time to first use of the limb, days to dynamization, days to ESF removal, radiographic evaluation, clinical outcome, and owner satisfaction ([Table 1]).

Table 1 Data summary

CCase	Signalment	Initiating Trauma	Description of fracture	Surgical procedure	ESF configuration	Weight of frame and surgery time	First use of limb (days after surgery)	Complication	Dynamization and ESF removal (days after surgery)	Clinical outcomes
1	2-year-old, 4 kg, DSH, ♂	HRI	Tibia, closed, mid-diaphysis, transverse	Closed fashion	EHF: P:2 × 2.0 mm, D:2 × 2.0 mm (3:negative threaded, 1:non threaded)	27.2 g, 25 min	1	–	–, 32	FFO
2	4-year-old, 3.6 kg, DLH, ♂	HRI	Humerus, closed, mid-distal diaphysis, comminuted, nonunion	Open fashion	EHF: P:2 × 2.0 mm + 1 × 1.5 mm, D:2 × 1.5 mm + 1 × 2.0 mm (6:negative threaded)	21.4 g, 85 min	3	–	42, 63	AFO
3	1-year-old, 3 kg, DSH, ♀	HRI	Tibia, closed, mid-diaphysis, oblique	Closed fashion	EHF: P:2 × 2.0 mm, D:2 × 1.5 mm (2:negative threaded, 2:non threaded)	27.2 g, 25 min	2	–	–, 28	FFO
4	1-year-old, 4.5 kg, DSH, ♂	DB	Femur, closed, mid-diaphysis, transverse	Open fashion	EHF: Tie-in P:2 × 2.0 mm, D:2 × 1.5 mm Int:1 × 2.0 mm (3:negative threaded, 1:non threaded)	20.5 g, 35 min	1	Mild pin-tract infection	–, 35	FFO
5	3-year-old, 4.5 kg, DSH, ♂	HRI	Antebrachium, closed, mid-diaphysis, transverse	Limited open fashion	EHF: P:2 × 1.5 mm, D:2 × 1.5 mm (2:negative threaded, 2:non threaded)	38.8 g, 30 min	1	Mild pin-tract infection	21, 35	FFO
6	2-month-old, 750 g, DSH, ♀	UKN	Femur, closed, mid- diaphysis, spiral	Closed fashion	EHF: Tie-in P:2 × 1.5 mm, D:2 × 1.5 mm, Int:1 × 2.0 mm (4:negative threaded)	13 g, 15 min	1	–	10, 16	FFO
7	1-year-old, 3.6 kg, DLH, ♂	HRI	Tibia, type 1 open, mid-diaphysis, comminuted	Limited open fashion	EHF: P:2 × 2.0 mm + 1 × 1.0 mm, D:2 × 2.0 mm + 1 × 1.0 mm (2:negative threaded, 4:non threaded)	28.2 g, 30 min	3	Osteomyelitis, delayed union	21, 49	AFO
8	4-month-old, 1.6 kg, DSH, ♀	UKN	Tibia, closed, distal diaphysis, oblique	Closed fashion	EHF: P:2 × 2.0 mm, D:2 × 1.0 mm (2:negative threaded, 2:non threaded)	26.2 g, 20 min	1	–	–, 21	FFO
9	7-month-old, 2.5 kg, British Shorthair, ♂	HRI	Tibia, closed, mid-diaphysis, oblique	Closed fashion	EHF: P:2 × 2.0 mm, D:1 × 1.5 mm, 1 × 2.0 mm (2:negative threaded, 2:non threaded)	27.2 g, 25 min	1	–	21, 28	FFO
10	3-month-old, 1 kg, DSH, ♂	HRI	Tibia, closed, distal metaphysis, comminuted	Closed fashion	EHF: P:2 × 1.5 mm, D:2 × 1.5 mm (3:negative threaded, 1:non threaded)	27.2 g, 25 min	1	Mild pin-tract infection	–, 18	FFO
11	5-month-old, 2 kg, DLH, ♀	HRI	Tibia, closed, distal diaphysis, oblique	Closed fashion	EHF: P:1 × 2.0 mm + 1 × 1.5mm D:2 × 1.5 mm (2:negative threaded, 2:non threaded)	27.2 g, 25 min	1	–	–, 21	FFO
12	8-month-old, 3 kg, DSH, ♂	HRI	Tibia, type 1 open, distal diaphysis, comminuted	Closed fashion	EHF: P:2 × 2.0 mm, D:2 × 1.5 mm (3:negative threaded, 1:non threaded)	32.8 g, 35 min	2	Implant Failure	21, 28	FFO
13	1-year-old, 3.8 kg, DSH, ♂	DB	Femur, closed, mid-diaphysis, transverse	Closed fashion	EHF: Tie-in P:1 × 2.0 mm, D:2 × 1.5 mm, Int:1 × 2.0 mm (2:negative threaded, 1:non threaded)	20 g, 20 min	1	Mild pin-tract infection	–, 28	FFO
14	11-month-old, 3.7 kg, British Shorthair, ♀	HRI	Tibia, closed, proximal metaphysis, comminuted	Closed fashion	EHF: P:1 × 2.0 mm + 1 × 1.5 mm, D:3 × 2.0 mm (2:negative threaded, 3:non threaded)	27.2 g, 25 min	1	–	21, 35	AFO
15	1-year-old, 5 kg, DSH, ♂	UKN	Antebrachium closed, mid-diaphysis, comminuted	Closed fashion	EHF: P:2 × 1.5 mm, D:2 × 1.5 mm (2:negative threaded, 2:non threaded)	42.5 g, 30 min	1	Mild pin-tract infection	–, 35	FFO
16	13-year-old, 3 kg, DSH, ♀	HRI	Tibia, closed, mid-diaphysis, transverse	Closed fashion	EHF: P:2 × 2.0 mm, D:1 × 1.5 mm + 1 × 2.0 mm (2:negative threaded, 2:non threaded)	27.2 g, 25 min	1	Mild pin-tract infection	21, 49	FFO
17	1-year-old, 4.8 kg, Siamese, ♂	HRI	Tibia, type 1 open, mid-diaphysis, comminuted	Closed fashion	EHF: P:3 × 2.0 mm, D:2 × 2.0 mm + 1 × 1.5 mm (5:negative threaded, 1:non threaded)	32.8 g, 20 min	1	–	21, 35	FFO
18	10-month-old, 3 kg, DLH, ♂	HRI	Tibia, closed, distal diaphysis, comminuted	Closed fashion	EHF: P:2 × 2.0 mm, D:1 × 1.5 mm + 1 × 1.0 mm (2:negative threaded, 2:non threaded)	27.2 g, 25 min	1	–	–, 28	FFO
19	11-month-old, 3 kg, DSH, ♂	UKN	Femur, closed, proximal diaphysis, oblique	Closed fashion	EHF: Tie-in P:1 × 2.0 mm, D:2 × 1.5 mm, Int:1 × 2.0 mm (3:negative threaded)	20 g, 20 min	1	–	–, 28	FFO
20	9-month-old, 3 kg, DSH, ♂	HRI	Tibia, closed, proximal metaphysis, comminuted	Closed fashion	EHF: P:2 × 1.0 mm, D:2 × 2.0 mm (2:negative threaded, 2:non threaded)	27.2 g, 30 min	1	–	–, 28	FFO
21	1-year-old, 3.3 kg, Persian, ♂	HRI	Humerus, closed, supracondylar, intercondylar, comminuted	Open fashion	EHF: P:2 × 2.0 mm, D:1 × 1.5 mm + 1 × 1.0 mm (3:negative threaded, 1:non threaded)	28.2 g, 50 min	1	–	–, 28	FFO
22	1-year-old, 7.3 kg, DSH, ♂	HRI	Tibia, closed, distal Metaphysis, transverse	Closed fashion	EHF: P:2 × 2.0 mm, D:1 × 1.5 mm + 1 × 1.0 mm (2:negative threaded, 2:non threaded)	32.8 g, 30 min	1	–	21, 35	FFO
23	3-year-old, 5.2 kg, DLH, ♂	UKN	Antebrachium, closed, distal diaphysis, oblique	Limited open fashion	EHF: P:2 × 1.5 mm, D:2 × 1.0 mm (2:negative threaded, 2:non threaded)	40.8 g, 35 min	1	–	–, 42	FFO
24	11-month-old, 3.5 kg, DSH, ♂	HRI	Tibia, closed, distal metaphysis, transverse	Closed fashion	EHF: P:1 × 1.5 mm+ 1 × 2.0 mm D:1 × 2.0 mm + 1 × 1.0 mm (1:negative threaded, 3:non threaded)	27.2 g, 25 min	1	–	21, 32	FFO
25	3-year-old, 6 kg, DSH, ♂	HRI	Femur, closed, distal diaphysis, oblique	Closed fashion	EHF:Tie-in P:1 × 2.0 mm, D:2 × 2.0 mm + (2:negative threaded, 1:non threaded)	18 g, 25 min	1	–	–, 35	FFO
26	2-year-old, 4 kg, DSH, ♂	UKN	Tibia, type 2 open, distal diaphysis, comminuted	Limited open fashion	EHF: P:3 × 2.0 mm+ 1 × 1.5 mm D:3 × 2.0 mm + 2 × 1.0 mm (5:negative threaded, 4:non threaded)	32.8 g, 35 min	3	Osteomyelitis, nonunion	–, 35	UAO
27	7-month-old, 2.5 kg, DSH, ♂	HRI	Femur, closed, mid-diaphysis, comminuted	Open fashion	EHF:Tie-in P:1 × 2.0 mm, D:2 × 2.0 mm + (1:negative threaded, 2:non threaded)	20 g, 35 min	2	–	–, 28	FFO
28	1-year-old, 5 kg, British Shorthair, ♂	HRI	Tibia, type 1 open, mid-diaphysis, spiral	Limited open fashion	EHF: P:1 × 2.0 mm + 1 × 1.5 mm, D:1 × 1.5 mm + 1 × 1.5 mm (2:negative threaded, 2:non threaded)	32.8 g, 25 min	2	Mild pin-tract infection	21, 32	AFO
29	2-year-old, 5.5 kg, DSH, ♂	HRI	Femur, closed, mid-diaphysis, comminuted	Limited open fashion	EHF:Tie-in P:1 × 1.5 mm, D:2 × 2.0 mm + (1:negative threaded, 2:non threaded)	18 g, 30 min	1	–	–, 35	FFO
30	3-year-old, 3.5 kg, DSH, ♀	UKN	Antebrachium, closed, mid-diaphysis, comminuted	Limited open fashion	EHF: P:2 × 1.5 mm, D:2 × 1.5 mm (2:negative threaded, 2:non threaded)	41.8 g, 35 min	2	–	21, 42	AFO

Abbreviations: AFO, acceptable functional outcome; D, distal fragment; DB, dog bite; DLH, domestic longhair; DSH, domestic shorthair; EHF, exvet hybrid fixator; FFO, full functional outcome; g, gram; HRI, high rise injury; Int, intramedullary pin; P, proximal fragment; UAO, unacceptable outcome; UKN, unkown; ♀, female; ♂, male.

Implant Description, Features, and Frame Design

Exvet small ESF system consisting of 40-mm and 50-mm diameter full, 3/4, 1/4 modified rings, 52-degree modified arches, 4-mm diameter carbon fiber connecting rods (30, 50, 80, 100, 120, 150 mm lengths), 3 mm diameter grub screws, cubic devices, adaptors, 2- and 4-mm exballs, universal covers, 1.5 and 2.0 mm diameter negative profile end-threaded pins, 1- and 1.5-mm diameter Kirschner wires, and 2.0-mm smooth pins ([Fig. 1]). The connecting rods were made of carbon fiber, transfixation pins were made of 316-L stainless steel, and the other components of the system were made of 7050 aluminum alloy. Surgeries were performed with a custom-designed frame for each case.

Fig. 1 Instrumentation for the Exvet fixator system and its components. A 50-mm diameter 90-degree semicircular arch (A), 4-mm diameter carbon fiber connecting rods (B), 2.0-mm clamp (C), 2-mm exball with universal cover (UC) (D), 40-mm diameter 270-degree ring (E), cubic device (F), 50-mm diameter 270-degree ring (G), 40-mm diameter 90-degree semicircular arch (H), 50-mm diameter 52-degree modified semicircular arch (I), 3-mm diameter grub screw (J), Allen key (K), 4-mm exball with modified adapter (L), pin-insertion point (asterisk) and rod insertion point (arrowhead).

The design of the rings and arches included several holes. Half of these holes can be used for rods and pins insertion, whereas the other half can be used for locking pins and rods to the ring with a grub screw. Transfixation pins, ranging from 0.6 to 2.2 mm in diameter and carbon fiber rods, up to 4 mm in diameter can be fixed to modified rings, arches, and cubic devices using grub screws via the threaded holes of the rings ([Fig. 2]).

Fig. 2 Application of Exvet hybrid frame. Pin insertion into the bone independently (A) and frame placement to the pin through the arch hole (B, C). A 3-mm diameter grub screw placement to lock the pin (arrowhead), and 3-mm diameter grub screw placement to lock the carbon fiber rod (red arrow) (D). After inserting the two Kirschner wires into the bone via ring holes (E), additional pin insertion through the clamp (F) to complete the hybrid frame (G).

Versatile use options of the transfixation pins are possible for Exvet rings, arches, and cubic devices. Additionally, multiplanar and 360-degree unlimited pin placement options are available for these implants with the use of universal cover and 2-mm diameter exballs ([Fig. 1D]). However, to achieve 360-degree unlimited connecting rod placement options, the universal cover, 4-mm diameter exball, and adaptor must be used together with rings or arches ([Fig. 1L]).

Surgical Procedure

Anesthetic management, perioperative antibiotic management, and perioperative pain management were conducted according to standard protocols at each institution. The preoperative planning included frame configuration, considering the cat's body condition and fracture type, pin or wire diameter, as well as the assessment of radiographs of the fractures.

When a limited open surgical approach was performed, temporary intramedullary pins up to 2.0 mm in diameter were used in either a retrograde or normograde fashion to achieve adequate alignment of the main bone fragments, without further disruption of the fracture hematoma and soft tissues. After fracture alignment, soft tissue and skin closure, the custom-designed frame was applied. Subsequently, the intramedullary pin was either removed or connected to the frame for a Tie-in configuration ([Fig. 3]). All cases with open fractures were debrided and lavaged with 0.9% saline solution prior to ESF application. The technique used for closed reduction in the majority of fractures was the hanging limb technique, except for femoral and humeral fractures. In the cases referred to us for surgical revision after complications, adequate fracture reduction was performed while trying to preserve the formed callus.

Fig. 3 A 2-month-old, 750 g Domestic shorthair cat presented with a 3-day history of lameness due to mid-diaphyseal spiral femoral fracture (A, B). An intramedullary pin was placed in a normograde fashion to improve axial alignment of the two main bone fragments without disrupting the fracture hematoma. After the ESF application, the intramedullary pin was connected to the frame in a Tie-in configuration (13 g) (C, D, E, F). The intramedullary pin was removed 10 days after surgery to provide early dynamization. Radiographs showed a moderate callus at the fracture site 16 days after surgery (G, H); thus, the fixator was removed. The final radiographic follow-up occurred 2 weeks after ESF removal (I, J). ESF, external skeletal fixation.

In all cases, pins and Kirschner wires were inserted in a “far–far–near–near” fashion. The first two pins were placed into the proximal and distal fracture segments as far away as possible from the fracture lines and secured to the frame. After achieving an acceptable reduction of the fracture, the third and fourth pins were placed into the proximal and distal fracture segments as close as possible to the fracture line. Additional transfixation pins were then inserted to complete the construct.

Postoperative Clinical Evaluation

Cats were assessed daily for limb use while hospitalized. Reexaminations occurred in each case 10 days after surgery and were repeated every 10 days until fixator removal. During reexaminations, the tightness of the grub screws and clamps was checked. Owners were advised to clean the pin–skin area with an antiseptic solution and to keep the cats indoors without cage rest until the removal of the fixator. When a reexamination was missed, the owner was contacted by phone to report the cat's condition.

Clinical outcomes and complications were evaluated based on the criteria proposed by Cook et al as follows: full function—the ability to maintain activities at the same level and duration as before the injury. Acceptable function—requires medical treatment to maintain the preinjury state. Unacceptable function—any other outcome. Complications were classified as major or minor. Major complications required additional surgery or treatment, whereas minor ones resolved without further treatment.[21] [23]

Postoperative Radiographic Evaluation

Radiographic follow-up examinations were performed immediately after surgery and every 10 days until ESF removal in all cases. Additionally, the final radiographic follow-up examinations occurred 9 months after ESF removal in some cases. Modified Yang's scoring system was used to assess callus formation grade and pin loosening grade at the time of ESF removal.[23] [24] According to this scoring system, the pin–bone interface was graded on a scale from 0 to 2 points. Grade 0 was defined as a completely radiolucent interface, grade 1 as a partially radiolucent interface, and grade 2 as fully integrated. Each pin was graded individually and recorded. Callus formation was graded as follows: none (0), minimal (1), moderate (2), and abundant (3).

Results

Signalment, Cause of Fracture, and Bone Distribution

Thirty cases met the inclusion criteria. Breeds included Domestic shorthair (n = 20), Domestic longhair (n = 5), British shorthair (n = 3), Siamese cat (n = 1), and Persian cat (n = 1). Eighteen of the 30 cats were intact males, 5 were castrated males, 5 were intact females, and 2 were spayed females. The mean age was 23.5 months (range: 2 months to 13 years), and the mean body weight was 3.7 kg (range: 750 g to 7.35 kg).

Fracture etiology included high-rise syndrome (n = 21), unknown trauma (n = 7), and dog bites (n = 2). Five (16.6%) were open fractures. Four open fractures were classified as type 1 and one as type 2 according to the AO principles of fracture management.[25] Seventeen (56.6%) of the fractures were tibial, 7/30 (23.3%) were femoral, 4/30 (13.3%) were antebrachial, and 2/30 (6.6%) were humeral ([Fig. 4] and [Videos 1], [2]). Seven (23.3%) were transverse, 9/30 (29.9%) were oblique or spiral, and 14/30 (46.6%) were comminuted. Additionally, 24/30 (79.9%) were diaphyseal, 5/30 (16.6%) metaphyseal, and 1/30 (3.3%) was intra-articular.

Fig. 4 A 1-year-old, 3.3 kg Persian cat was referred for the treatment of a left intra-articular distal humeral fracture following surgical complication (A, B). A transcondylar screw and a hybrid ESF frame (28.2 g) were placed via a medial open surgical approach (C, D, E, F). The ESF frame was removed 4 weeks after surgery, and radiographic follow-up examinations revealed bony union (G, H). Additionally, flexion and extension angles of the elbow joint were within normal limits, and the cat achieved a full functional outcome. The final radiographic follow-up occurred 5 months after ESF removal (I, J). ESF, external skeletal fixation.

Video 1 Gait video of the Persian cat with an intra-articular distal humeral fracture (Case No: 21), recorded 6 hours after surgery.

Video 2 Gait video of the Persian cat (Case No: 21), recorded immediately after ESF removal. ESF, external skeletal fixation.

Surgical Data

Nineteen fractures (63.3%) were fixed in a closed fashion ([Fig. 5]) (14 were tibial, 4/19 were femoral, 1/19 was antebrachial). Seven (23.3%) were fixed in a limited open surgical fashion (three were tibial, 1/7 was femoral, 3/7 were antebrachial) without disturbing the fracture site. Four (13.3%) were fixed in an open fashion (two were femoral, and 2/4 were humeral) ([Fig. 6]). The time between the first incision or pin placement and the first suture or last pin placement was considered the surgery time. The median surgery time was 30 minutes (range: 15–85). Revision surgeries were performed in three cases (9.9%) (three were tibial open fractures).

Fig. 5 Preoperative radiographs of the right tibia and fibula of a 1-year-old, 4.8 kg Siamese cat showed a mid-diaphyseal comminuted type 1 open fracture (A, B). An ESF frame (32.8 g) was applied via closed fashion and orthogonal radiographic views confirmed adequate alignment and reduction of the fracture (C, D, E, F). To increase the working length of the pins, two grub screws near the fracture line were removed without sedation 3 weeks after the surgery. Thus, staged disassembly was achieved to provide fracture healing. The ESF frame was removed 2 weeks after dynamization, and radiographic evaluation showed complete union of the fracture (G, H). The cat achieved full functional outcome immediately after removal. The final radiographic follow-up occurred 5 months after ESF removal (I, J). ESF, external skeletal fixation.

Fig. 6 A 4-year-old, 3.6 kg, Domestic longhair cat was referred for the treatment of a left diaphyseal humeral nonunion fracture (A, B). The sequestrum was excised, and the bone fragments were debrided. Following autogenous bone grafting, a hybrid ESF was applied (C, D, E, F). To increase the working length of the pins, one pin near the fracture line was removed from the distal fragment 6 weeks after surgery. Radiographic follow-up 3 weeks after dynamization revealed bony union (G, H), and the ESF frame (21.4 g) was subsequently removed. The final radiographic evaluation was performed 2 months after ESF removal (I, J). ESF, external skeletal fixation.

Frames consisted of 2 rings/arches and 2 or 3 clamps for femoral, humeral, tibial, and antebrachial fractures. The mean number of pins for each frame was 4.2 (range: 3–6). The diameter of transfixation pins was chosen as described by Corr.[26] Ten of 30 (33.3%) frames included 3 carbon fiber connecting rods, and one rod was removed in all 10 cases 21 days after the surgery to provide early dynamization, as described by Schultz et al.[27] ([Fig. 7]). In two cases (6.6%), dynamization was achieved by increasing the working length of the pins, and in one case (3.3%), by removing the intramedullary pin. Seventeen of 30 (59.9%) frames included two carbon fiber connecting rods and dynamization was not performed ([Fig. 8]).

Fig. 7 An 11-month-old, 3.7 kg British shorthair cat presented with a nonarticular proximal metaphyseal comminuted tibial fracture (A, B). A Hybrid ESF (27.2 g) was applied via closed fashion, and immediate postoperative radiographs showed adequate alignment and reduction of the fracture (C, D, E, F). An additional connector rod was removed from the cranial surface of the frame 21 days after the surgery to provide early dynamization. Although radiographic follow-up examination revealed grade 1 callus formation according to the Modified Yang's scoring system and mild valgus deformity, the ESF frame was removed 2 weeks after dynamization (G, H). The cat achieved an acceptable functional outcome immediately after ESF removal. According to the owners, the cat achieved full functional outcome 3 weeks after ESF removal. Radiographic follow-up evaluations at 4 and 9 months after ESF removal demonstrated bone remodeling consistent with Wolff's law (I, J, K, L). ESF, external skeletal fixation.

Fig. 8 A 4-month-old, 1.6 kg Domestic shorthair cat presented with a distal 1/3 diaphyseal oblique tibial fracture (A, B). A hybrid ESF (26.2 g) was applied via closed fashion (C, D). Orthogonal radiographic views confirmed adequate alignment and reduction of the fracture (E, F). Three weeks after surgery, radiographs showed moderate callus formation at the fracture line (G, H), and the fixator was subsequently removed. The final radiographic follow-up occurred 6 months after ESF removal (I, J). ESF, external skeletal fixation.

Postoperative Data

Complications occurred in 10 (33.3%) cases, with 7 considered minor and 3 major. All minor complications were due to mild pin-tract infections and were resolved with cleaning of the pin–skin interface, antibiotic administration, and ESF removal. Major complications occurred in open fractures, and surgical intervention was required. Of these, one was due to frame configuration, unrelated to the open fracture and was resolved with additional pin insertion in a closed fashion. Osteomyelitis and sequestrum formation occurred in two of three cases. These two were treated with antibiotics, sequestrum excision, and placement of antibiotic-impregnated polymethylmethacrylate beads. One of these two ended with a delayed union. In the other cat, leg amputation had to be performed.

On control radiographs, 9 of 126 (7.1%) pins exhibited a completely radiolucent bone/pin interface (grade 0 modified Yang's score). Twenty-six of 126 (20.6%) pins were scored as grade 1, and 91 of 126 (72.2%) pins were grade 2. Callus formation was scored immediately after ESF removal. One of 30 (3.3%) cases was grade 0, 5 of 30 (16.6%) cases were grade 1, 20 of 30 (66.6%) cases were grade 2 ([Fig. 8]), and 4 of 30 (13.3%) cases were grade 3.

Twenty-two cats started weight bearing immediately after surgery ([Videos 3] [4] [5] [6]), and 8 within 1 to 3 days. Radiographic evaluations showed bony union in 29 of 30 cases. The mean time to the final radiographic follow-up was 92 days (range: 14–270). Immediately after ESF removal, full functional outcome was achieved in 24 (79.9%) cases, 5 (16.6%) had an acceptable outcome ([Videos 7], [8]), including one delayed union case, and 1 (3.3%) case ended with amputation.

Video 3 Gait video of the Siamese cat with a mid-diaphyseal comminuted type 1 open fracture (Case No: 17), recorded 4 hours after surgery.

Video 4 Gait video of the Siamese cat (Case No: 17), recorded immediately after ESF removal. ESF, external skeletal fixation.

Video 5 Gait video of the Siamese cat (Case No: 17), recorded 2 weeks after ESF removal. ESF, external skeletal fixation.

Video 6 Gait video of the Siamese cat (Case No: 17), recorded 6 months after ESF removal. ESF, external skeletal fixation.

Video 7 Gait video of the Domestic longhair cat with a diaphyseal humeral nonunion fracture (Case No: 2), recorded 6 weeks after surgery.

Video 8 Gait video of the Domestic longhair cat (Case No: 2), recorded 5 weeks after ESF removal. ESF, external skeletal fixation.

Discussion

Biological osteosynthesis is an attractive concept in fracture management. The strategies of biological osteosynthesis are used when the surgeon determines before surgery that anatomical reconstruction of the multiple cortical fragments is unlikely to result in load sharing. In these highly comminuted fractures, better results can be achieved by applying a set of biological osteosynthesis strategies in which preservation of vascular supply to the damaged bone is balanced with rigid fracture fixation.

Fracture healing is a natural process; therefore, surgical interventions should be performed in a minimally invasive manner to preserve fracture hematoma, soft tissues, and vascular supply to promote fracture healing.[14] [16] Minimally invasive osteosynthesis is honoring all these principles and is feasible in dogs and cats.[9] [23] [28] ESF with closed reduction is one of the best biological osteosynthesis methods to spare soft tissue damage.[14] [16] In the present study, 26 fractures of 30 cats were stabilized in a minimally invasive fashion introducing a new ESF frame and functional outcomes were evaluated.

Others reported transarticular ESF use for the management of tarsocrural laxity in 32 cats, with a complication rate of 41%.[29] In another study, ESF was used for the treatment of tibial fractures in cats with 50% complications.[6] In our study, the complication rate related to ESF was 33.3%, which is lower compared with the other reports.[13] [23] [30] Similar to other studies, the majority (70%) of complications we saw were considered minor and included pin-tract infection. Pin-tract infection is the most common complication in ESF application and may result in early pin loosening. Other reasons for pin loosening include incorrect pin insertion, thermal necrosis, excessive patient activity, and increased stress at the pin–bone interface caused by biomechanical properties.[31] Pin loosening occurred in 9 (7.1%) of the 126 pins in this study. All 9 pins were located close to the joints; thus, all were associated with excessive motion and pin-tract infection.

In one study, the median time to removal of transarticular ESF for treatment of tarsocrural laxity in 32 cats was reported to be 46 days, whereas another study reported a median removal time of 49 days for resin–acrylic bar ESF used in treating long-bone fractures in cats.[1] [29] The median ESF removal time appears to vary based on the aim of the surgery, fracture type, age, breed, and surgical approach. In this study, the mean age was 23.5 months, and 63.3% of fractures were fixed in a closed fashion. The mean time to ESF removal was 32 days (range: 16–63), which could be considered relatively short. In most cases, the criteria for frame removal were based on clinical signs and not necessarily on radiographic evidence of complete bone healing.

Intraoperative imaging offers several advantages, including improved reduction in closed surgeries and accurate pin insertion.[7] However, despite these benefits, studies have shown that correct fracture alignment can be achieved without it.[9] [23] Although intraoperative imaging was not used in this study, no immediate revision surgeries were required in any case. Additionally, percutaneous needles were used to determine joint lines and ensure correct pin placement in tibial and antebrachial fractures, without the use of fluoroscopic guidance.

In circular and hybrid systems, hexagonal nuts are one of the most important components of the frame.[13] [22] [32] [33] [34] Nuts can only work with threaded rods; however, threaded carbon fiber rods are not being produced due to the nature of carbon fiber material. In this study, carbon fiber rods were able to function in both hybrid and circular configurations due to the use of grub screws instead of nuts. The tightness of the grub screws was reevaluated in each case 10 days after discharge and monitored regularly until fixator removal. While all grub screws were secure during the initial 10-day evaluation, some were found to have loosened by the second 10-day assessment. This loosening was attributed to increased clinical activity in the cats. The use of carbon fiber rods decreases the weight of the frame, which is crucial for small animals in ESF applications, the frame has to be as light as possible while being strong enough to stabilize bone fragments.[19] [35] [36] Additionally, carbon fiber rods facilitate X-ray evaluation due to radiolucent properties. In this study, only carbon fiber rods were used, and the frames weighed 13 to 42.5 g without transfixation pins, and the total weight was 14.8 to 50.1 g when pins and attachments were included.

There is evidence that the staged disassembly of the ESF frame may enhance fracture healing by stimulating the osteogenic response during the healing process.[27] [31] [37] [38] [39] [40] One study has reported that early axial dynamization promotes callus formation and remodeling, compared with control groups.[37] In this study, dynamization was performed 3 weeks, 6 weeks, and 10 days after surgery in 11, 1, and 1 cases, respectively. However, the effect of dynamization on callus formation could not be assessed statistically, radiographically, or clinically due to the lack of uniformity in the ages, fracture types, and localizations of the cases. Additionally, control groups were not included in this study. Although there is no clinical data, dynamization is believed to promote fracture healing according to the author's clinical experience.

This study had some limitations. All closed reductions were performed through manual manipulation of the fractures without intraoperative imaging assessment, which did not affect the outcomes; however, fluoroscopic evaluation could have shortened surgery time. If closed reduction could not be achieved within 15 minutes, a limited open surgical approach was preferred to avoid potential damage to soft tissues and bone fragments during the attempt at closed reduction. All surgeries were performed at two different institutions by different senior surgeons (S.U., A.D., C.Y.) who have experience in ESF application. Therefore, techniques and case management may have varied in some cases.

In conclusion, minimally invasive ESF application is an effective treatment for long-bone fractures in cats. Although the postoperative complication rate is higher due to minor complications, the short- and long-term outcomes were not affected. We described a novel ESF system with good outcomes and high owner satisfaction. The Exvet fixator appears to be an effective, practical, and versatile ESF that does not require intraoperative fluoroscopic assessment. Therefore, this novel ESF system could be considered an innovative alternative to other systems for managing long-bone fractures in cats.

Conflict of Interest

S.U. is the inventor and main shareholder of the Exvet System and has applied for a patent. However, he does not receive any funds or royalties from the company. The authors and Exvet Medical declare that there is no conflict of interest.

Authors' Contributions

S.U. is the primary author who planned, designed, and wrote the work, developed the implants, and supervised all procedures. A.D., M.Y.D., and F.Q. contributed to case management and manuscript preparation. C.Y. contributed to the review and editing of the manuscript.

Ethics Statement

This study does not present any ethical concerns. In addition, for all diagnostic and medical procedures, an informed consent of the owner was obtained.

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Publication History

Received: 26 May 2024

Accepted: 04 March 2025

Article published online:
01 April 2025