Articulated Intramedullary Pin-Hybridized Circular External Skeletal Fixator Construct for Stabilization of Comminuted Femur Fractures in Three Cats

Abstract

This case series describes the clinical results following stabilization of comminuted diaphyseal femoral fractures, two of which extended into the proximal metaphyseal region, using an articulated intramedullary (IM) pin-hybridized circular external fixator construct in three cats. An IM pin was placed to initially align and stabilize each fracture via an open reduction. The fixator construct consisted of a proximal partial ring arch and a distal stretch ring. Half-pins were used as fixation elements on the proximal ring, whereas olive wires were used as fixation elements on the distal ring. All three fractures were acceptably reduced and aligned. The constructs afforded functional use of the limb during the post-operative convalescent period with few complications. The constructs were maintained for 8, 15, and 16 weeks until there was sufficient radiographic fracture healing for fixator removal. In one cat, the IM pin was maintained following fixator removal and was articulated with a single retained half-pin engaging the greater trochanter and femoral neck. All three cats were reported to have acceptable pelvic limb function at 12, 25, and 93 months following surgery, with two of the cats reported to have occasional, intermittent nominal lameness.

Keywords

circular external skeletal fixator - cats - comminuted diaphyseal femur fracture - hybridized

Introduction

Circular external skeletal fixation is a useful modality for stabilizing comminuted appendicular long bone fractures, particularly juxta-articular fractures, in dogs and cats.[1] [2] [3] [4] Traditional circular constructs employ small-diameter wires as fixation elements.[3] [5] [6] [7] [8] The wires are secured at opposing positions around the ring's circumference, limiting their application for proximal extremity fractures.[3] [5] [9] Circular constructs can be hybridized by using larger-diameter full- or half-pins instead of small-diameter wires, enabling stabilization of proximal limb fractures.[10] [11] [12] This case series describes use of a hybridized circular construct articulated with an intramedullary (IM) Steinmann pin to stabilize comminuted diaphyseal femoral fractures, two extending into the subtrochanteric region, in three cats.

Case Series Descriptions

Three male neutered cats were presented to the University of Florida Small Animal Hospital (UF-SAH) between January 2017, and August 2024, for femoral fracture stabilization. Cat #1 was a 52-month-old, 4.1 kg domestic shorthair; Cat #2 an 84-month-old, 5.4 kg domestic medium-hair; and Cat #3 an 11-month-old, 4.9 kg Siamese. Cat #1 and Cat #3′s injuries occurred in the home, whereas Cat #2's injury was sustained outdoors. Lameness onset was acute, and the inciting event was not observed. On physical examination, all cats had non-weight-bearing lameness of the right (Cat #1 and #3) or left (Cat #2) pelvic limb, with thigh swelling, variable pain, and femoral instability on palpation. No neurological abnormalities were identified.

Pre-operative radiographs revealed diaphyseal femoral fractures with soft tissue swelling in all three cats. Cat #1 had a comminuted spiral proximal diaphyseal fracture of the right femur extending proximolaterally to the third trochanter, with caudal and proximal displacement ([Fig. 1]). Cat #2 had a large segmental diaphyseal left femur fracture with caudolateral displacement of the distal femoral segment. The proximal margin was transverse in the subtrochanteric region, and the distal margin of the central fracture segment was comminuted with a fissure extending proximally ([Fig. 2]). Cat #3 had a highly comminuted mid-diaphyseal right femoral fracture with substantial caudoproximal displacement ([Fig. 3]).

Fig. 1 Craniocaudal and mediolateral preoperative radiographs of Cat #1's right proximal femoral fracture which extended laterally to involve the third trochanter (A and B). Photograph of Cat #1's articulated intramedullary pin-hybridized circular construct that consisted of a proximal partial ring arch with three half-pins as fixation elements and a distal stretch ring with two opposing olive wires. Caudolateral and craniolateral-to-craniomedial threaded rods were used as connecting elements (C). Initial postoperative radiographs confirming acceptable alignment and stabilization of the fractured femur (D and E). Craniocaudal and mediolateral radiographs of Cat #1's right femur obtained 15 weeks following surgery prior to (F and G) and immediately after fixator removal (H and I).

Fig. 2 Craniocaudal and mediolateral preoperative radiographs of Cat #2's left proximal femoral fracture, which extended into the subtrochanteric region (A and B). Initial postoperative radiographs confirming acceptable alignment and stabilization of the fractured femur (C and D). Craniocaudal and mediolateral radiographs of Cat #2's left femur obtained 14 weeks following surgery prior to (E and F) and immediately after fixator removal with the intramedullary pin and a single proximal half-pin retained (G and H). Craniocaudal and mediolateral radiographs obtained at 16 weeks following surgery showing fracture of the retained proximal half-pin with the intramedullary pin still in place (I and J) and immediately following removal of the intramedullary pin leaving the intraosseous segment of the broken half-pin (K and L).

Fig. 3 Craniocaudal and mediolateral preoperative radiographs of Cat #3′s right comminuted mid-diaphyseal femoral fracture (A and B). Initial postoperative radiographs confirming acceptable alignment and stabilization of the fractured femur (C and D). Photograph of Cat #3′s articulated intramedullary pin-hybridized circular construct 8 weeks' post-operative that consisted of a proximal partial ring arch with three half-pins as fixation elements and a distal stretch ring with two opposing olive wires. Caudolateral and craniolateral-to-craniomedial threaded rods were used as connecting elements (E). Craniocaudal and mediolateral radiographs of Cat #3′s right femur obtained 8 weeks following surgery prior to (F and G) and immediately after fixator removal (H and I).

The cats were anesthetized and placed in lateral recumbency with the unaffected pelvic limb dependent. A lateral approach was made to the affected femur extending from the greater trochanter to slightly beyond the distal extent of the fracture. Bone-holding forceps were applied to the proximal and distal fracture segments. A 2.0-mm Steinmann pin was placed in normograde fashion into the trochanteric fossa and advanced into the proximal medullary canal. After exiting the proximal femoral segment, the pin tip was excised to blunt the implanted end. The fracture was aligned, and the pin was advanced into the distal segment and seated in the distal metaphysis. Fluoroscopy confirmed reduction, alignment, and implant placement.

Cancellous bone graft from the ipsilateral proximal humerus was placed in all fracture sites. Cancellous autograft was combined with commercial allogenic bone (Osteoallograft Periomix Feline, Veterinary Transplant Services, Inc, Kent, Washington, United States) in Cat #1. Three circumferential #1 PDS sutures were placed, incorporating minimal soft tissue, to approximate the butterfly, proximal, and distal fracture segments in Cat #1.

Components from the IMEX Circular External Skeletal Fixation System (IMEX Veterinary, Inc., Longview, Texas, United States) were used for all three constructs. Two slightly divergent, opposing lateral-to-medial 1.6-mm olive wires were placed in the distal femoral metaphysis, perpendicular to the distal femoral frontal anatomic axis, and secured to a 66-mm stretch ring positioned proximal to the stifle, with the open section oriented caudally. Three divergent 1.6/1.9 mm, 2.0/2.3 mm, or 2.4/2.9 mm Miniature Interface half-pins (IMEX Veterinary, Inc., Longview, Texas, United States) were inserted into the lateral surface of the proximal femur and directed toward the femoral neck or subtrochanteric region at slightly different longitudinal levels. The fixation pins were attached to a 66-mm incomplete ring arch. The two rings were articulated using connecting elements, including a caudolateral and craniolateral-to-craniomedial diagonal rods to support the stretch ring medially. The IM pin was bent adjacent to where the pin protruded through the skin and articulated to the fixator ([Figs. 1C] and [3E]). The ipsilateral coxofemoral and stifle joints were palpated post-placement to ensure an acceptable range of motion.

Post-operative radiographs were evaluated to assess fracture reduction and femoral alignment ([Figs. 1D] and [E], [2C] and [D], [3C] and [D]). Adjustments to the fixator were made to address femoral head and neck normoversion in Cat #3. Triple antibiotic ointment was applied around the fixation element–insertion sites, and sterile gauze was placed over the wounds. The construct interior was loosely packed with dried surgical sponges and wrapped with a soft padded bandage using cast padding, Kling, and self-adhesive bandage wrap (Vetrap, 3M, Saint Paul, Minnesota, United States). All three cats recovered uneventfully from anesthesia and surgery.

Peri-operative analgesia and antibiotics included robenacoxib (1 mg/kg per os [PO]) once daily; buprenorphine (0.1 mg/kg transmucosal) three times daily; gabapentin (10 mg/kg PO) three times daily; and cephalexin (30 mg/kg PO) twice daily. Trazodone (5 mg/kg PO) twice daily was administered for anxiolysis in Cat #2. Bandaging was applied to the construct and changed 7 days following surgery. Bandaging was discontinued 2 weeks after surgery, and a loose cloth shroud was secured around the fixator.[13]

At-home care instructions included daily cleaning around the fixation element–skin interfaces with 0.05% chlorhexidine, followed by application of triple antibiotic ointment and replacement of a clean shroud. Owners were instructed to confine their cat to limit activity. All three cats tolerated the construct and placed substantial weight on the affected limb, with minor lameness at the 4-week evaluation. None of the cats developed pin- or wire-tract complications. There was a progressive increase in weight-bearing and overall mobility.

Radiographs obtained 4 and 8 weeks postoperatively showed maintained alignment, no implant failure, and bridging callus formation by 8 weeks in all three cats. Callus formation was substantial enough for fixator removal at week 8 in Cat #3. Follow-up radiographs at 12 and 15 weeks for Cat #1 documented continued consistent fracture healing, with sufficient bridging callus to allow fixator removal at 15 weeks ([Fig. 1F] and [G]). Post-fixator removal radiographs confirmed osseous union ([Fig. 1H] and [I]). Additional follow-up radiographs were obtained at 12 and 14 weeks for Cat #2. By week 14, the fracture had nearly obtained union ([Fig. 2E] and [F]), and the fixator was removed; however, the IM pin was retained and articulated with a single remaining proximal half-pin ([Fig. 2G] and [H]) due to the cat's high activity level and concern for potential re-fracture. At 16 weeks postoperatively, the owners of Cat #2 returned to UF-SAH for evaluation due to new-onset lameness and displacement of the retained IM pin. The proximal half-pin had broken at the lateral pin–bone interface ([Fig. 2I] and [J]). The extracorporeal broken segment of the half-pin and IM pin were removed, while the implanted segment of the broken half-pin was left in situ ([Fig. 2K] and [L]). Fracture healing was sufficient that no further stabilization was warranted.

Anatomical femoral alignment was retrospectively assessed based on post-fixator removal radiographs of the fractured femur and compared with pre-operative radiographs of the contralateral femur ([Table 1]). Alignment discrepancies between the affected and unaffected femur were < 10 degrees. Paired t-tests (n = 3 for each measurement) showed the mean anatomical lateral proximal femoral angle (aLPFA) was significantly greater in the healed femur (p = 0.007). Discrepancies in anatomical lateral distal femoral angle (aLDFA) and anatomical caudal proximal femoral angle (aCdPFA) between limbs were not significant (p = 0.184, 0.635; [Table 1]).

Table 1 Measurements of femoral sagittal and frontal plane alignment

	Healed femur			Contralateral femur			Discrepancy
Cat #	aLPFA	aLDFA	aCdPFA	aLPFA	aLDFA	aCdPFA	aLPFA	aLDFA	aCdPFA
1	103 degrees *	87 degrees	33 degrees	97 degrees *	94 degrees	29 degrees	6 degrees *	7 degrees	4 degrees
2	115 degrees *	90 degrees	38 degrees	107 degrees *	90 degrees	31 degrees	8 degrees *	0 degrees	7 degrees
3	110 degrees *	86 degrees	21 degrees	103 degrees *	93 degrees	26 degrees	7 degrees *	7 degrees	5 degrees
Mean ± SD	109.3 ± 6 degrees *	87.7 ± 2.1 degrees	31 ± 9 degrees	102.3 ± 5 degrees *	92.3 ± 2.1 degrees	29 ± 3 degrees	7 ± 1 degrees *	4.7 ± 4 degrees	5 ± 2 degrees

Notes: Measurements of sagittal and frontal plane alignment of the healed femur (based on immediate post-fixator removal radiographs) and contralateral intact femur in three cats that had femur fractures stabilized with an articulated intramedullary pin-hybridized circular external fixator construct. Parametric paired t-test, with a 95% confidence interval, yielded p-values of 0.007, 0.184, and 0.635, for aLPFA, aLDFA, and aCdPFA. An asterisk indicates measured parameter which had a significant difference between the healed and contralateral femur. Anatomical lateral distal femoral angle (aLDFA), anatomical lateral proximal femoral angle (aLPFA), anatomical caudal proximal femoral angle (aCdPFA).

Cat #2 was solicited to return to the UF-SAH for an evaluation 25 months after surgery. The owners reported full functional limb use, with only occasional, nominal weight-bearing lameness, and the owners were highly satisfied with surgical outcome. On examination, Cat #2 was ambulatory without lameness and had full range of motion in the ipsilateral coxofemoral and stifle joints. Goniometric measurements revealed flexion and extension of the affected hip to be 30 and 171 degrees, compared with 27 and 170 degrees in the contralateral hip. Stifle flexion and extension measured 25 and 165 degrees in the affected limb, compared with 30 and 160 degrees in the contralateral limb. Muscle mass was comparable, with the thigh circumference of the affected limb measuring 19.9 cm and the contralateral limb 20.1 cm. Radiographs revealed no change in the location of the portion of the fixation pin embedded in the greater trochanter and femoral neck. The healed femur had remodeled, with a 2 degrees aCdPFA discrepancy compared with the contralateral femur. The aCdPFA was 5 degrees closer to the contralateral limb than the measurement taken immediately after fixator removal, indicating less femoral recurvatum. No discrepancies were noted in aLPFA or aLDFA.

The owners of Cat #1 and #3 were contacted by telephone at 12 and 93 months post-operatively but declined to return for evaluation. Both owners reported full functional limb use, although the owners of Cat #1 reported occasional, nominal weight-bearing lameness. Both owners were highly satisfied with the cat's post-operative outcomes. All three cats were perceived to have appropriate activity levels and quality of life, without pain or disability.

Discussion

This case series validates the efficacy of using an articulated IM pin-hybridized circular external skeletal fixator construct for comminuted diaphyseal femoral fractures in three cats. Circular constructs are advantageous for fractures with short, juxta-articular segments, allowing placement of multiple divergent fixation elements.[10] [11] [14] [15] [16]

Two fractures extended into the subtrochanteric region, a location subjected to complex mechanical forces, necessitating stable fixation to prevent malunion, delayed union, or nonunion.[17] The IM pin was placed to provide axial alignment and distract the femur to approximate normal length.[18] [19] [20] The pin also afforded some inherent stability, primarily resisting bending forces. Application of the fixator provided supplemental stability, addressing axial, torsional, and bending forces.[21] [22]

The IM pin diameter was <50% of the femoral medullary canal, allowing for placement of the half-pins and olive wires. The portion of the pin protruding through the skin was bent 90 degrees and articulated with the proximal ring arch to increase construct stiffness.[21] Linear external skeletal fixator-IM pin tie-in constructs have been successfully used to stabilize femoral fractures, achieving union without implant migration and with minimal morbidity.[21] Independent IM pin placement combined with circular constructs has been reported to promote early limb use and recovery.[3] [11] [23] [24] Use of an IM pin articulated with a semi-circular construct has been reported to stabilize humeral fractures in dogs, but their application for proximal limb fractures in cats has not been previously described.[12]

Circular constructs have primarily been used to stabilize antebrachial and crural fractures; however, application of circular constructs using wires as fixation elements in the proximal limb are limited by anatomical constraints.[3] [6] [25] The hybridized construct in this case series employed partially threaded half-pins to stabilize the proximal femoral segment.[26] [27] An incomplete ring arch served as a platform for three divergent Miniature Interface half-pins placed in the lateral cortex and directed toward the femoral neck and subtrochanteric region at varying longitudinal levels to optimize osseous engagement and limit soft tissue impedance.

Deformation of incomplete rings reduces construct stiffness; however, placing a complete ring to secure the distal femur is impractical.[4] [28] Deformation and the decrement of construct stiffness with the use of incomplete rings are inversely proportional to ring diameter, and the 66-mm stretch rings did not impair healing.[15] Opposing olive wires were used as fixation elements on the stretch rings to limit translation of the distal segment, as divergency of these wires was limited to mitigate entrapment of the quadriceps and other muscles.[6]

Use of the articulated IM pin-hybridized circular construct resulted in acceptable restoration of femoral frontal and sagittal plane alignment. Alignment was assessed on post-fixator removal radiographs using measurement techniques described in dogs.[29] [30] Measurements included the aLPFA, aLDFA, and aCdPFA, and compared with the contralateral femur. Only aLPFA differed significantly, indicating 6- to 8-degree femoral varus. A prior study showed that angular differences < 10-degrees fall within normal anatomical variation and are unlikely to compromise function or require correction.[31]

All three cats tolerated the construct well, having an early return to weight-bearing and progressive improvement in mobility. The weight of the construct did not appear to impair limb function or the cats' mobility. Owners were able to perform daily pin and wire tract care and shroud changes without difficulty.

Cat #2's fixator was removed at 14 weeks, although union was somewhat tenuous and the owner's compliance with activity restriction was poor. The IM pin and one half-pin were retained and connected with a linear rod to support remodeling. The half-pin broke 2 weeks later, consistent with prior reports of delayed mechanical failure in deconstructed constructs under continued load-bearing.[21] [32] [33] Radiographs showed maintained alignment and continued remodeling, so the IM pin and accessible half-pin segment were removed. Fracture healing proceeded uneventfully, as confirmed at the 25-month evaluation. The main limitations of this report are its retrospective nature and the small number of cases. A large prospective case series with a control population would better assess the efficacy of the construct in managing femoral fractures in cats. Additional limitations include the lack of comprehensive, objective outcome measures, such as force plate gait analysis or validated client questionnaires.[34] [35] While long-term radiographs and functional assessments, such as goniometric measurements and limb circumference, were obtained in one cat, standardized data acquisition from all cases was unavailable. Despite these constraints, the positive results described support the use of articulated IM pin-hybridized circular constructs as a viable alternative to plate fixation, particularly for proximal fractures not amenable to traditional plating options or an interlocking nail.[36] [37] [38] [39]

Conclusion

The outcome of these cases illustrates the utility of an articulated IM pin-hybridized circular construct for stabilizing comminuted proximal femoral fractures in three cats. While previous studies have shown the effectiveness of traditional circular constructs for distal limb fractures in cats and dogs, their application in proximal limb fractures has been limited by anatomical constraints. Placement of an IM pin with the fixator mirrors established techniques used in distal limb fractures, where IM fixation enhances biomechanical stability.[23] Prior studies have described hybrid linear-circular constructs to stabilize proximal juxta-articular fractures, although the segment was located distally in nearly all cases.[10] [11] [14] [15] [16] Biomechanical evaluations to optimize construct design and application may further improve outcomes and guide broader implementation.

Conflict of Interest

None declared.

Animal Care Guidelines

Client consent was obtained for all procedures performed, and all care provided was consistent with best practice standards of veterinary medicine.

Author Contributions

L.M.S. and D.D.L. examined and diagnosed the patient, performed surgeries, performed follow-up visits, analyzed data, and wrote and revised the manuscript. B.M.B. analyzed medical images and data, performed long-term follow-up, and wrote and revised the manuscript.

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Publication History

Received: 29 July 2025

Accepted: 01 October 2025

Article published online:
31 October 2025