34th Annual Scientific Meeting proceedings

Veterinary cementless THR systems were developed about 30 years ago, in part to overcome the mid-to long-term complications associated with cement such as aseptic loosening. Initial stability is obtained by bone-implant friction and long-term stability results from osteointegration of the implant which is usually expected, at least partially,  within a few months following surgery. Revision of an implant that is not yet osteointegrated can be easily achieved via extraction, modification and re-implantation. When the implant is osteointegrated, revision surgery, including extraction is significantly more challenging.

Revision of the osteointegrated cup
Extraction of an osteointegrated cup risks significant damaging of the bone bed and acetabular pillars which can prevent stable reimplantation. Several strategies have been described to address hip luxation to avoid explanting an osteointegrated cup. Triple pelvic osteotomy has been described to address dorsal luxation¹. This procedure reduces the angle of lateral opening (ALO), and possibly increases the version angle (VA). Monoti et al reported a good to excellent outcome in 12/18 dogs, major complications in 6/18 dogs (4 luxations and 2 deep infections) and a poor long-term outcome in 2 dogs despite an absence of peri-operative complication¹. Double pelvis osteotomy has also been described by Thibault et al for correction of dorsal luxation with cemented cups². However, this technique was associated with a higher rate of short-term re-luxation (5/11) and a high rate of long-term explantation (7/11)  because of cup loosening (5/11) or infection (2/11). Chantziaras et al described the surgical management of ventral luxation via a ventral augmentation of the acetabulum with a plate or/and a polyethylene implant³. Although this technique does not correct any cup orientation and impingement issues, they reported a return to full function in 7/9 dogs and a good outcome in 8/9.

Luxation is often due to a combination of excessive ALO, an inappropriate version, and/or excessive inclination of the cup. Triple and double pelvic osteotomy can only correct a limited number of implant positioning-related complications which limits its application to selected cases and ventral acetabulum augmentation does not address any impingement problem which may be a concern in the long term. An alternative is to extract the osteointegrated press-fit cup with a combination of osteotome and burr and replace it with a new one. This allows to correct misplacement in the 3 plans (ALO, VA, IA) while still being able to re-implant a new cup.

We describe a case series where all acetabular cups (8 BFX BioMedtrix, 1 Helica) were successfully extracted with a new cup being re-implanted (9/9 BFX). In 7 cases, BFX cups were revised with a larger cup. Eight cases had minimal lameness at 6 weeks postoperatively; 1 dog experienced repeat luxation diagnosed at the 6-week recheck which resolved after open reduction and hobble placement. All cases had minimal lameness and all cups were radiographically stable at the last recheck follow-up (median 45 days, 35-618). Medium-long-term outcome (mean 885 days, sd ±600) was graded as excellent or good in all 6 dogs this was available for. One dog represented 18 months post revision with a low-grade infection resulting in cup loosening and partial bone resorption around the proximo-medial part of the stem. Histology of peri-articular tissues was also compatible with metallosis. Whether metallosis was clinically significant and has been a predisposing factor to the infection or is an incidental finding is unknown. Authors strongly recommend protecting the peri-acetabular tissues with moist swabs to limit the contamination with metallic fragments during the burring of the cup.

Revision of the osteo-integrated stem
There is only one case report in the literature of a fractured BFX stem (cobalt chrome) revision and none on the new BFX titanium electronic beam melted stem⁴. In that case report, an extended V-shape trochanteric osteotomy was performed and a new BFX stem re-implanted. Despite a few complications such as stem subsidence and the need for a second stem revision with reimplantation of a bigger (2 sizes up) collared BFX stem, the final outcome was satisfactory.

Using a relatively similar approach to the stem, we describe the results of 3 BFX EBM lateral bolt stem fracture revisions. The three stems could be successfully extracted via a femoral window technique with extended trochanteric osteotomy^4,5,6. The osteotomy was fixed with 4 double-loop cerclage wires of 1.2mm,  a cemented CFX stem was re-implanted and the repair was augmented with a LCP plate with locking screws placed laterally. Short-term recovery was good for all 3 dogs. One dog fractured its femur and plate 9 months post-operatively and the THR was explanted at the time of femoral fracture repair, one dog was euthanised 13 months after the revision surgery because of a gastric tumour and the third case did not have any complications 18 months post revision surgery. Femoral and plate fracture 9 months post-operatively likely resulted from incomplete healing of the femoral window osteotomy. This may be due to some interposition of cement between the bone window and the femoral shaft associated with an absence of a bridging callus. Re-implanting a BFX stem instead of a CFX stem may limit the risk of this medium to long-term fracture as well as aseptic loosening.

An alternative and indirect revision approach can be performed on an osteointegrated stem that sustained some mild to moderate degree of subsidence associated with an unacceptable retroversion. Instead of opening the femur to extract the stem, the stem version issue can be corrected using a transverse femoral osteotomy and rotation of the distal femur that is subsequently fixed with an LCP plate. This technique can only be applied if the degree of stem subsidence does not affect the hip laxity or the range of motion after correction because of a too-short femoral offset.

Conclusion
Although explanting the last generation of osteointegrated EBM BFX implants is challenging, when performed carefully a re-implantation is possible and allows good to excellent outcomes. A bigger case series is warranted to determine if stem revision with a CFX stem is the best option or if re-implantation with a BFX stem should be preferred.

References

1. Monotti I, | Ryan S, Preston C, Management of total hip replacement luxation with triple pelvic osteotomy, Veterinary Surgery. 2018;47:993–1001.
2. Thibault A, Haudiquet PH, Poor success rates with double pelvic osteotomy for craniodorsal luxation of total hip prosthesis in 11 dogs Veterinary Surgery. 2023;52:1219–1227
3. Ventral acetabular augmentation for management of caudoventral luxation following total hip replacement in dogs Chantziaras V, Pink J, Kulendra E et al. Journal of Small Animal Practice (2022) 63, 34–44
4. Petazzoni M, De Giacinto E, Marcellin-Little D, et al Revision of a Press Fit Biological Fixation Stem Fracture in a Dog VCOT Open 2019;2:e19–e26.
5. Peck JN, Marcellin-Little DJ. Revision strategies for total hip replacement. In: Peck JN, Marcellin-Little DJ, eds. Advances in Small Animal Total Joint Replacement. West Sussex, UK: Wiley; 2012:119–121
6. Dyce J, Olmstead ML. Removal of infected canine cemented total hip prostheses using a femoral window technique. Vet Surg 2002;31:552–560