34th Annual Scientific Meeting proceedings

Advanced degenerative disease of the femoro-patellar compartment of the knee joint is a common and challenging problem in small animals. Arthrotic changes of the femoral trochlea and the patella occur as a result of various mechanical disorders of the knee joint. Commonly they arise in patients with patella luxation or chronic patella maltracking syndrome (PMTS), in patients with chronic anterior cruciate ligament disease, as a result of previous intra-articular fractures or in cases of various angular limb deformities. Many times joint destruction is iatrogenic as a result of unsuccessful previous surgeries. A non functional cartilage on the femoral trochlea is found in patients with congenital trochlear aplasia and 4th degree of patellar luxation. In these cases, degenerate and atrophic trochlea cartilage is observed due to lack of a previous mechanical contact between femoral joint surface and the patella. Dogs with high grade patella luxation are commonly presented with asymmetric cartilage wear of the trochlea ridge. In these cases,  substantial cartilage loss and subchondral bone eburnation can be observed and is probably a significant source of pain. Patello-femoral arthroplasty has gained increasing importance in human and veterinary orthopedics in the past few years. Causes of patello-femoral osteoarthritis in humans are basically similar to those in our patients. Traditionally, various techniques of trochleoplasties were performed in small animals to manage PMTS with trochlear hypoplasia. Although the result may lead to a realignment between the trochlea and the patella, in many cases they are associated with further progression osteoarthritis and long term function is often disappointing. One important reason is a lack of healthy cartilage in the femoral trochlea in many of this cases. Even after correction of underlying limb deformities which are a common cause of PMTS, generally a gradual progression of degenerative disease of the femoro-patellar joint is observed.

In humans, hemiarthroplasty of the knee joint has been performed in various forms for the past 50 years. In 2015, Dokic et al described the first clinical application of a unicompartmental endoprosthesis, patella groove replacement (PGR, Kyon CH) in in dogs on a small case series. The two component implant (groove and base plate) and the original technique of implantation has its limitations and risks. Therefor a new trochlear prosthesis, the Trochlear Ridge Arthoplasty (TRA), was developed recently and was released 2022. The implant is a single piece press fit prosthesis which can be fixed with only two optional screws. The trochlear implant is made of titanium alloy (Ti6Al4) and its surface is specially hardened with a layer of amorphous diamond-like carbon. This layer makes it possible to reduce the coefficient of friction and is supposed to be resistant to mechanical wear. The profile is more anatomical in respect to a normal femoral trochlea. There are 13 different sizes of TRA implants available at the moment. 

Surgical technique: A trochlea ostectomy is performed as described by Dokic et al. starting distally in the proximal direction, removing most of the trochlea and creating a perfectly flat surface. The correct size and position of the prosthesis is confirmed by placing a trial implant fixed with two temporary pins to the bone. If patella tracking is smooth the trial implant is replaced by a drill guide using same positioning pins. A central hole is reamed through the guide to accomodate the press-fit plug of the prosthesis which is porous for better osteoconduction. The final implant is then impacted and finally fixed with two screws (1.5mm, 2.4mm or 2.7mm depending the size of the implant). 

Soon after the TRA prosthesis a new prototype of a pure press-fit implant was introduced to clinical trials (starting from 2023). The Intrauma MM prosthesis has a very anatomical shape, with low profile with a distal part to the prosthesis extending below the level of long digital extensor tendon partially into the intecodylar fossa. This allows a patella-prosthesis contact even in full flexion.  The single piece implant is a pure press fit prosthesis ( 3-4 pegs) with no screws. Ten sizes are currently available. A very important difference is the implantation technique. The trochlea is not removed by a saw but with the use of specific guides and cannulated reamers. The distal part of the femur is prepared manually with a dedicated rasp.

Even though both systems are very different, in case of a complications one system can be used as a revision option for the other, because the fixation points do not interfere. The author has revised two TRA cases with the MM prosthesis. In one case we converted after technical problems with a MM prosthesis to the TRA prosthesis in the same procedure. Also the PGR system can be revised with MM or TRA and it was performed by the author on several occasions.

Both new prosthetic systems eliminate some negative aspects of the established resurfacing prosthesis PGR. In contrast to other available trochlear prostheses the MM prosthesis and TRA are single piece press fit implants, so implant luxation is unlikely. Only two screws are used in TRA, no screws with MM, therefore the prosthesis  does not interfere with other implants on the distal femur, including very small patients. In our study group no impingement between patella and the implant was observed as it is described  by Dokic at al.  Also all prostheses were stable at time of writing, no patella re-luxation was observed in cases of previous PMTS.

However the prosthesis itself should not be considered as the primary method of stabilization for patella luxation in most cases. Also, the possibilities of compensating for angular deformities using a TRA prosthesis are limited. External or internal femoral torsion can be safely compensated up to approx. 6-8 °. Compensation of excessive femoral varus or valgus deformity is possible up to 5 ° only. All deformities beyond that values need to be addressed first with corrective osteotomies.

Outcome evaluation is difficult as is the comparison between the systems. Most cases have multi level corrections like femoral or tibial osteotomies (or both) or tibial tuberosity transposition - depending the indication. Most patients are showing satisfactory limb function (lameness I-III / VI) about 7 to 10 days after removal of the bandage, depending on other accompanying procedures. In patients (were available ) the biometrical gate analysis (Gate4dogs pressure sensitive walkway) was near normal or indistinguishable from normal eight weeks after the procedure. Only dogs with massive osteoarthritis and soft tissue involvement (atrophy, contractures, etc.) show mild lameness for more than 8 weeks. Over 4 months, the majority is asymptomatic and fully functional. Outcome measures are complicated however, due to the heterogenic character of patients by size, complementary procedures and indications.

For TRA and the MM prosthesis the implantation technique as such is relatively simple, but it must be emphasized, that even very minimal deviations in the placement of the prosthesis can lead to serious complications with the patella tracking. In addition, it is obvious that the majority of patients also require complex corrections of angular deformities, which are often technically demanding and, moreover, difficult to perform without access to advanced imaging like CT. In contrast to prostheses applied in humans, the TRA (as other veterinary hemiprostheses) resurface only the femoral trochlea site not the patella surface. So far there are no objective data evaluating long term influence on the patella surface and its cartilage wear.