Use of a Spacer to Increase Soft Tissue Tension in a Feline Total Hip Replacement

Abstract

A 3-year-old male neutered cat was referred for management of a chronic femoral capital physeal fracture. Significant secondary remodelling changes appreciated on radiographs included calcar bone resorption and intertrochanteric trabecular sclerosis.

A total hip replacement was performed utilizing BioMedtrix total hip replacement implants. A number 10-mm BFx cup was implanted in an appropriate orientation. Given the loss of calcar bone, the osteotomy of the femoral neck was positioned further distal than ideal. Trial reduction of the monoblock femoral stem (number 2 +4 mm) revealed excessive joint laxity. A 4-mm BioMedtrix tibial plateau leveling osteotomy spacer was placed under the collar of the femoral stem to proximalize the stem, thereby resulting in appropriate tension across the joint.

Follow-up radiographs at 8 weeks and 6 months postoperatively showed good osseous integration of the acetabular cup and a stable stem. The cat exhibited no hindlimb lameness or discomfort during hip manipulation. Hindlimb muscle symmetry and left hip range of motion indicated restored limb function.

Keywords

total hip replacement - feline - capital physeal fracture

History and Physical Examination Findings

A 3-year-old neutered male domestic Shorthair cat (4.20 kg) presented for evaluation of a progressive, weight-bearing lameness in the left pelvic limb. The cat had no outdoor access, and no traumatic event was reported by the owner. The owner first noticed the lameness 2 weeks prior to presentation. The primary care veterinarian had started the cat on gabapentin 50 mg orally every 12 hours for pain control, with exercise restriction. Minimal clinical improvement was noted by the owner, and the cat was referred to our facility.

On physical examination, the cat exhibited a grade 3/5 lameness in the left pelvic limb, with significant atrophy of the left thigh and gluteal muscles and pain localized to the left hip joint. Assessment revealed a significant reduction in range of motion, pain and mild crepitus in the left hip joint; the remainder of the orthopaedic examination was unremarkable. The physical examination was otherwise within normal limits, except for an elevated body condition score of 6/9.

Diagnostic Findings and Interpretation

Haematology and chemistry panel were largely unremarkable, aside from a mildly elevated globulin level (5.6 g/dL; reference range 2.8–5.1 g/dL). Sedated pelvic radiographs revealed a chronic left capital physeal fracture characterized by intertrochanteric sclerosis, marked calcar bone resorption and mild coxofemoral osteoarthritic changes ([Fig. 1]). These findings were consistent with chronic remodelling secondary to the capital physeal fracture. The femoral neck demonstrated extensive cortical resorption, corresponding to a grade 3 osteolysis score according to the grading systems established by McNicholas and colleagues.[1]

Fig. 1 Mediolateral (A) and craniocaudal (B) preoperative femoral radiographs. The black arrow indicates the region of significant calcar resorption and the red arrows indicate the intertrochanteric sclerosis.

Treatment and Outcome

Continued medical management was deemed unlikely to be successful, and surgery was recommended. Treatment options included femoral head and neck ostectomy (FHO), total hip replacement (THR) and amputation. Ultimately, the owner opted for THR surgery to maximize the chances of a full return to pain-free joint function.

At surgery, a number 10-mm BFx cup was implanted with a satisfactory angle of lateral opening, version and inclination ([Fig. 2]). Significant bone resorption was present in the femoral neck, extending from the subphyseal bone distally to the calcar region. Given the loss of bone in the calcar region of the femur, the osteotomy of the femoral neck was positioned further distally than ideal ([Fig. 3]). The femoral canal was prepared for a number 2 nano THR monoblock cemented femoral stem/head implant. Due to the distal location of the osteotomy, trial reduction of the monoblock femoral stem/head with the longest commercially available neck (number 2 +4 mm) revealed excessive laxity across the joint, permitting easy luxation of the prosthesis. Given that +4 mm is the longest neck length available in the monoblock, options included finding a way to increase tension across the joint, or we could have converted to an FHO.

Fig. 2 Orthogonal radiographic views of the left femur ([A] craniocaudal, [B] mediolateral) and of the pelvis ([C] frog-legged, [D] lateral) acquired immediately postoperatively. Note the distal position of the femoral osteotomy necessitating distal femoral stem positioning. To overcome excessive laxity across the joint, a 4-mm BioMedtrix TPLO spacer was placed between the collar of the stem and the osteotomy line. The 4-mm measurement is the proximal-to-distal height of the spacer. A single-loop cerclage was used to help secure the spacer.

Fig. 3 Craniocaudal (A) and mediolateral (B) femoral radiographs acquired 6 months postoperatively. Note the cement void that is apparent around the tip of the stem. The radiographic distribution of this void suggests possible lateral and caudal translation of the stem tip during the final stages of cement curing.

In an effort to increase soft tissue tension across the joint, a 4-mm BioMedtrix ‘Patellign’ TPLO (tibial plateau leveling osteotomy) spacer was inserted between the femoral stem and the osteotomy of the femoral neck to proximalize the stem, thereby increasing tension across the joint ([Figs. 3] and [4]). Prior to cementing, the construct was trialled to confirm adequate soft tissue tension across the coxofemoral joint. Bone cement was injected into the femoral canal during the late liquid phase using a 25-mL syringe and a syringe extension tube.[1] [2] After the spacer and stem were introduced and the cement had cured, a single-loop cerclage wire was applied around the proximal femur and the spacer, utilizing a groove in the side of the spacer to further secure the spacer in place ([Figs. 3] and [4]). Postoperative stem alignment was deemed adequate; the stem was tilted mildly into varus (by 3.8 degrees) and caudally (4.3 degrees). The femoral anteversion was also deemed to be slightly greater than preferred ([Fig. 3]).

Fig. 4 Picture of a 4-mm (green line) BioMedtrix ‘Patellign’ TPLO spacer. This spacer is designed to be placed under the distal portion of a TPLO plate when the plate is applied to a medialized proximal tibial segment as part of a TPLO-M correction technique in dogs with medial patellar luxation. The encircling cerclage wire was placed in the groove in the spacer (white arrow).

Comments

Spontaneous capital physeal femoral fracture is a well-recognized non-traumatic condition that occurs in young, overweight male cats.[1] Fracture fixation can be attempted in cats that are diagnosed early, prior to any significant remodelling changes to the proximal femur. In more chronic cases like the one presented here, the surgeon should consider either FHO or THR. Total hip replacement is currently the gold standard treatment for hip disease, with evidence of better return to function when compared with FHO.[2]

In this case, bone remodelling and resorption in the proximal femur resulted in insufficient tension following standard femoral preparation and stem placement. Maintenance of appropriate soft tension across the joint seems to be especially important in small-breed THR patients, where coxofemoral luxation is reported to be the most common postoperative complication, representing approximately 16% of complications in one study.[3] While one possible solution would have been to raise the stem higher within the cement bed without using a spacer, this would have resulted in a reduced implant–cement interface area and would have created a longer lever arm for the portion of the unsupported implant proximal to the cement, thus increasing bending forces at the stem–cement interface. These mechanical forces could have placed the cement mantle under excessive stress and increased the risk of aseptic stem loosening or stem fracture at the stem-cement interface. The spacer was selected as an alternative means to restore joint tension while minimizing strain at the cement interface, with the aim of improving the long-term stability of the construct.

An alternative approach would have been to perform the osteotomy at the routine level despite the underlying bone loss. This may have allowed for standard stem placement, though the medial collar extension of the implant would not have achieved direct contact with the calcar, potentially resulting in decreased proximal support. This option could be considered in similar cases, depending on the degree and distribution of bone loss. Additional potential strategies for managing inadequate coxofemoral soft tissue tension should also be considered. Cementless Micro-BFx stems, although not readily available at the time this surgery was performed, are now commercially accessible and provide an expanded range of stem sizes. This increased size selection may allow for improved implant fit and may help address soft tissue laxity in certain feline THR cases. Transposition of the greater trochanter has also been described as a technique to alter soft tissue orientation and improve tension, though it is a more invasive approach, and its efficacy in improving tension across the joint is not predictable.

In hindsight, we could have implanted a cup that was one size larger (number 12 BFx). This would have permitted the use of a modular micro THR stem and a larger femoral head (8 mm vs. 6 mm). The larger head would have reduced the risk of postoperative luxation by virtue of a larger jump distance, although the larger head likely would not have affected the soft tissue tension significantly.[4] The modular micro THR stem would have allowed us to use a longer neck (5 mm vs. 4 mm), thereby imparting more tension across the joint. However, given that we needed a 4-mm spacer to achieve appropriate tension, it is unlikely that the larger implant sizes would have provided sufficient soft tissue tension in this case.

This application of the spacer represents an off-label use of this implant, which was originally designed to be positioned beneath a TPLO plate as part of a modified tibial plateau leveling osteotomy correction ([Fig. 4]). In this case, the spacer was repurposed and placed circumferentially around the femoral stem to increase soft tissue tension in a THR. It is important to note that this alternative application has a number of potential concerns. First, there is a risk of fretting corrosion at the stem–spacer interface over time. Additionally, the spacer is not physically coupled to the stem, which limits its ability to effectively shield the proximal portion of the stem from axial or torsional loads. Over time, micromotion of the spacer could contribute to localized bone resorption or subtrochanteric osteolysis.

While this spacer offered a straightforward and readily available solution in this case, other strategies for addressing calcar bone loss or soft tissue laxity in THR include the use of custom femoral implants or structural bone grafting. While various options exist to manage proximal femoral bone loss in human THR, there does not appear to be a direct analogue to this particular type of modular spacer. This may reflect differences in anatomical considerations, implant design, or regulatory limitations in human medicine.

Given the cylindrical nature of the spacer, placing it on the distally sloping osteotomy surface inherently alters the axis of stem insertion. When the femoral stem is introduced through the spacer, the stem is not automatically oriented coaxially with the femoral diaphysis. To optimize alignment, we positioned the spacer with the open slot ([Fig. 4]) directed proximolaterally, which improved our ability to guide the stem along the native axis of the femur. Despite this, it is possible that the geometry and position of the spacer contributed to the mild varus and caudal angulation observed on the postoperative radiographs.

Follow-up examinations were conducted at 8 weeks and 6 months postoperatively, during which the owner reported the cat's comfort levels and function to be excellent. Radiographs acquired at both recheck visits showed good osseous integration of the acetabular cup, with no evidence of cup movement from the immediate postoperative radiographs. In addition, there was no appreciable stem or spacer movement, nor any other implant-related complications. On physical examination, the cat exhibited no lameness and was comfortable during hip manipulation. At the 6-month recheck visit, the cat walked with no discernible lameness and had no hip discomfort on manipulation. Thigh circumference measurements showed symmetry between the left (20.8 cm) and right (20.5 cm) limbs, indicating good restoration of strength and function in the left pelvic limb.

Ultimately, it is our hope that a wider range of implant sizes – particularly additional neck length options in the smaller stem sizes – will become available in the near future, reducing the need for intraoperative modifications such as those described in this case. That said, it is unlikely that a number 2 nano THR monoblock cemented stem–head implant could be manufactured with a neck extended by an additional 4 mm without significantly increasing the risk of fatigue failure at the neck–stem junction due to higher bending moments.

A primary limitation of this report is the lack of long-term follow-up, which precludes a full assessment of implant performance and durability over time. While the medium-term outcome in this case was satisfactory, the long-term consequences of using a non-coupled circumferential spacer in this location remain unknown. There is concern that this off-label application may contribute to accelerated cement mantle wear, micromotion at the stem–spacer interface and ultimately aseptic loosening. The reduced thickness of the cement mantle in the proximal femur – particularly where the spacer occupies space that might otherwise support cement – is one area that may be most predisposed to early mechanical failure or aseptic loosening, potentially necessitating revision surgery. Ongoing monitoring and additional case accumulation will be necessary to determine whether this technique is viable and safe in the long term. It is important to note that the 6-month follow-up period reported here represents only a short interval relative to the anticipated lifespan of this patient. As such, long-term implant behaviour in this context remains unknown. Aseptic loosening is a recognized complication in cemented THR and may be exacerbated by micromotion, uneven load distribution or degradation of the cement mantle over time.[5] Although no radiographic signs of implant loosening or osteolysis were observed at the 6-month recheck, these changes may develop gradually and warrant continued monitoring. Future evaluation of longer-term outcomes will be essential to assess the durability and safety of this technique. Without long-term follow-up, it would be premature to conclude that this technique is definitively safe. While the medium-term outcome in this case was favourable, extended observation is necessary to fully assess the risks and durability of this approach.

We hope that our approach in this case can provide some inspiration to other surgeons faced with a scenario similar to the one presented here. We conclude that the use of a spacer to proximalize a cemented nano THR stem was successful in reducing soft tissue laxity in the hip joint and resulted in acceptable short-term outcomes in a cat.

Conflict of Interest

None declared.

Disclosures

No AI-assisted technologies were used in the generation of this manuscript. The authors have nothing to disclose.

• References

• 1 McNicholas Jr WT, Wilkens BE, Blevins WE. et al. Spontaneous femoral capital physeal fractures in adult cats: 26 cases (1996-2001). J Am Vet Med Assoc 2002; 221 (12) 1731-1736
• 2 Liska WD, Doyle N, Marcellin-Little DJ, Osborne JA. Total hip replacement in three cats: Surgical technique, short-term outcome and comparison to femoral head ostectomy. Vet Comp Orthop Traumatol 2009; 22 (06) 505-510
• 3 Liska WD. Micro total hip replacement for dogs and cats: Surgical technique and outcomes. Vet Surg 2010; 39 (07) 797-810
• 4 Sariali E, Lazennec JY, Khiami F, Catonné Y.. Mathematical evaluation of jumping distance in total hip arthroplasty: Influence of abduction angle, femoral head offset, and head diameter. Acta Orthop 2009; 80 (03) 277-282
• 5 Bergh MS, Gilley RS, Shofer FS, Kapatkin AS. Complications and radiographic findings following cemented total hip replacement: A retrospective evaluation of 97 dogs. Vet Comp Orthop Traumatol 2006; 19 (03) 172-179

Publication History

Received: 28 April 2025

Accepted: 17 October 2025

Article published online:
28 November 2025