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Managing cranial cruciate ligament injuries is among the most common orthopedic problems seen by vets. Many techniques have been tried and these days it frequently comes down to a choice between these two (one is an intra-capsular procedure, the other extra-capsular).
The choice of surgical technique should really be left up to the veterinarian/surgeon. They will not only decide on the surgery they feel matches the individual animal, but perhaps most importantly, the surgery they are most experienced and comfortable doing.
The actual advantages (or disadvantages) of one or the other technique are also dependent on the expertise of the surgeon and the individual animal. Intra-articular techniques are said to mimic normal mechanics in that the graft should revascularize and resemble ligamentous tissue. This surgical option tends to be more intricate and challenging. The surgeon needs to be highly experienced and comfortable performing it.
According to extensive research published by Walter Renberg, DVM, MS, DACVS (Kansas State University), there is "no consensus on how to repair cranial cruciate ligament damage in the dog". No clinical studies have yet to indicate a better outcome with TPLO or TTA.
Find out what your veterinarian/surgeon prefers to perform and is most experienced with. His/her professional recommendation and input is essential.
When it comes to increased chances of arthritic complications, in 1996 (Chauvet et al) found no difference in outcome with either technique. All animals had arthritic progression. Another published study in 1996 (Jevens et al) compared the surgeries with no conclusions as to significant differences in outcomes. Recently a report (Conzemius) found no significant difference in the animals' performance.
These surgeries have been recorded as far back as 80 years (modified in the 1970's to include the 'over-the-top' option) and always being researched for improvements and studied for outcomes. I wish I could give you something more definitive than 'no significant difference', but there just isn't anything in published medical findings.
Arthritis appears to happen either way, the degree is more dependent on the particular animal and perhaps the expertise of the surgeon, but not the choice of surgery.
The injury was first described almost 80 years ago. The first known repair technique was proposed in 1952 by Paatsama. Since then, many techniques have been described/used. They are generally either intra- or extra-capsular procedures. The former are typified by the under and over technique. Among the latter are the fibular head transposition, tibial plateau leveling osteotomy, tibial tuberosity advancement, and various fabellar suture techniques.
In Practical Repairs of Cranial Cruciate Ligament Tears , Walter Renberg, DVM, MS, DACVS, of Kansas State University, writes: "Some techniques place sutures on both medial and lateral aspects of the joint, and some place sutures through one or two holes in the tibia while others use the patellar tendon. The fibular head transposition (FHT) was introduced in 1985 by Smith with additional publications investigating the procedure published later. Finally, Slocum introduced his cranial wedge osteotomy and later the tibial plateau leveling osteotomy (TPLO). The TPLO was a patented procedure which therefore required surgeons to attend a training course if they desired to perform the surgery. Unfortunately, the course had an excessively long waiting list for most practitioners. This fostered some negative feelings among the veterinary community. Additionally, the expense associated with the course and equipment, and the difficulty of the procedure made it less advisable for many veterinarians. The patent for the procedure has recently expired which may make the procedure more available. Similar procedures include tibial wedge osteotomies and tibial tuberosity advancement."
In his evaluation of advantages/disadvantages, Dr. Renberg adds: "The advantages to intra-articular techniques lie in the fact that they may recreate normal biomechanics better than other procedures and in the observation that the graft will revascularize to resemble ligamentous tissue. The biomechanical advantage has not been shown to correlate with decreased severity of arthritis or with higher performance. Additionally, the various intra-articular techniques are generally somewhat more challenging than a LFS."
Your companion obviously means a great deal to you and I understand that you want only the best for him so I appreciate you asking about this. Perhaps you will want to share this info with your vet. As far as I know, there's no more recent (or more importantly, definitive) studies or findings.