Key Points
In recent years, new total and hemi elbow replacement prostheses (TER & HER) have been designed (Conzemius, Acker – Van Der Muellen and Innes [TER] / Wendelburg – Tepic [HER]) for the treatment of intractable end-stage canine elbow osteoarthritis (OA). While most systems remain cemented, unlinked, semi-constrained, stemmed designs, a novel TER system (TATE Elbow™) was devised by Acker and Van Der Meulen. This proceeding, which focuses on the TATE Elbow™ prosthesis, describes recent modifications in implant design and surgical technique and summarizes clinical results and complications.
TATE design and surgical procedure
In addition to being an unlinked, semi-constrained design, the TATE is a cementless implant designed to use a novel resurfacing concept, as well as minimally invasive surgery (MIS). These fundamental differences may explain the improved, time-matched, clinical outcomes seen with the TATE prosthesis in particular with regards to lateral luxation and humeral/ulnar fractures, complications previously reported with cemented stemmed designs.
From a surgical standpoint, the elbow is approached via an osteotomy of the larger medial humeral epicondyle rather than a desmotomy of the lateral collateral ligament. In addition, the articular surfaces of the humerus, radius and ulna are simultaneously removed without luxating the elbow using a precision milling tool. This less invasive approach preserves both collateral ligaments and the majority of the osseous frame supporting the prosthesis, thus potentially enhancing postoperative stability, reducing morbidity, and hastening functional recovery. An alternative lateral approach has recently been proposed and shows promising clinical outcomes. Tone of the stated benefits of this approach is simpler patient positioning. Finally, CT based Patient Specific Instrumentations are being developed in an attempt to further improve surgical accuracy of critical steps including medial (or lateral) epicondyle osteotomy, identification of the joint center of rotation, milling depth and epicondyle reduction and fixation.
Accurate milling of the articular surfaces is a critical step to achieving the tight implant/bone interfaces required to optimize long-term secondary fixation via bone ingrowth. Milling accuracy has been improved through the development of a new, simpler instrumentation which included rigid temporary stabilization of the elbow, use of a magnetized milling arm and a 2-step milling process. As a cementless system, the TATE prosthesis is initially stabilized through a “press-fit” mechanism combined with the use of expendable primary fixation posts in the humerus, radius and ulna. Long term stability relies on bone ingrowth into the porous structure of the implants. In an effort to optimize osteointegration, the current, 3rd generation, TATE design includes hollow expandable primary fixation posts, hydroxyapatite coating of the Titanium Direct Metal Laser Melting prosthetic lattice surface. To reduce interfacial shear stresses and further promote osteointegration, articular congruity was decreased through flattening of the radioulnar articular profile. Finally. a vitamin E, highly crossed link UHMWE liner has been designed to improve longevity and reduce wear.
Both TATE components are impacted simultaneously as a pre-assembled cartridge. This unique characteristic guarantees accurate alignment and tracking of the prosthetic components throughout range of motion and likely reduces shear stresses at the bone-implant and articular component interfaces.
Clinical outcome
While clinical and experimental studies are ongoing at Michigan State University, no objective data is available on the clinical outcome of the TATE Elbow system. The following is a compilation of subjective and objective data from 3 clinical centers (Sun Valley Animal Center [Acker], MSU/TAMU [Authors] and Access Bone and Joint Center [Guiot]. We emphasize that this information is mostly subjective in nature and therefore should be assessed cautiously. Approximately fifty 3rd generation TATE prostheses have been implanted to date with 6 of these being through a lateral approach. Subjective clinical evaluation and feedback from dog owners suggest that limb function improves up to 1 year after surgery following a typical aggravation of the lameness between 6 and 12 weeks. Although dogs appear pain free and show improved range of motion, mainly in extension, subtle to mild lameness may persist. Severe complications including ulnar and humeral fractures, implant loosening and infection have been documented. Revision included primary repair, amputation and arthrodesis.
Objective force plate analysis was conducted on 10 patients at MSU and TAMU up to 8 years after implantation of TATE generations 1 and 2 designs. In all cases, pre-operative peak vertical force of the affected limb was significantly lower than normal reported range of 105% to 125% BW at the trot. By 6 to 12 months after surgery, the peak vertical force of the operated limb was greater than that of the contralateral side. Continued improvement was seen at 2 years, as the peak vertical force of the operated limbs had returned to a normal reported value of ~115% BW. One dog underwent bilateral elbow replacement. That dog received a 2nd generation TATE prosthesis 2 years after successful implantation of a 1st generation implant. In another case, polyethylene wear, resulting in metal-on-metal contact was documented 8 years after surgery. Peak vertical force was 85% BW on the operated leg and 120% BW on the untreaded side.
Regardless of design, a major limitation of TER is the absence of effective revision options in case of failure. Unfortunately, because end-stage elbow OA is often a bilateral condition, amputation is not a valid option in most cases and arthrodesis remains the main alternative. Although some fractures may successfully be revised, others may require explantation and arthrodesis because of the limited bone stock available for implant fixation. Infection is and will likely continue to be the most challenging complication as antibiotherapy alone is unlikely to be effective as long as the prosthesis is implanted. Because of these limitations, owner education is critical and must be thorough and objective. A fair disclosure of alternative treatments and realistic expectations, particularly with regards to complications and revisions, should be presented to anyone contemplating TER.