Cranial cruciate ligament disease (CrCLD) is a leading cause of lameness in the dog. Surgical treatment is advised for most dogs with CrCLD. A wide array of surgical techniques has been described for CrCLD dogs. They can be divided into static-constraint or dynamic-constraint (geometry-altering) methods. Two of the most commonly performed geometry-altering techniques are tibial plateau leveling osteotomy (TPLO) and tibial tuberosity advancement (TTA).

Biomechanical Comparison

It was originally suggested that the goal of TPLO was to level the tibial plateau slope (TPS) relative to the tibial long axis. However, it may well be that this method, like the TTA, reduces cranial tibial thrust by positioning the TPS 90° to the patellar tendon in the extended stifle position of the weight-bearing dog. If so, the TPLO moves the tibial plateau to meet the patellar tendon force, whereas for TTA, the force is moved to meet the tibial plateau. In either case, it appears that the end result is the same: the TPS and the patellar tendon become oriented at ~ 90° to each other. It has been suggested that the TPLO may tend to over-correct this goal. Conversely, in clinical practice and from review of in vitro mechanical studies, it appears that with currently available advancement instrumentation some dogs may be under-corrected using TTA. The under-correction, yet to be demonstrated, may be due to the variability in muscle co-contraction or in proximal tibial conformation among patients undergoing surgical treatment. Stifle conformation may also influence rotational stability, which is not statically provided by TPLO or TTA. However, reduction of tibial subluxation may reestablish a normal femorotibial alignment despite the presence of rotational laxity. Further studies needs to evaluate if in vivo dynamic stability can be achieved with TPLO or TTA.

Recent in vitro mechanical studies conducted at the University of Florida have shown that cranial tibial subluxation and internal tibial rotation associated with CrCL transection causes a caudal shift in femoro-tibial contact, reduced femoro-tibial contact area and associated increased peak contact pressures. When 90° patellar tendon-to-tibial plateau angle was obtained (this required mean advancement of 13.5 ± 1mm), TTA restored normal femoro-tibial contact parameters. Conversely, TPLO failed to restore normal femoro-tibial contact pressure patterns (femoro-tibial contact area remained smaller and peak contact pressures were positioned more caudally on the tibial plateau, when compared with CrCL intact stifles). These findings suggest that TPLO does not reestablish normal joint biomechanics following leveling the tibial plateau to 6°. Progression of osteoarthrosis in dogs treated by TPLO may be partly caused by an abnormal cartilage pressure distribution.

Surgical Comparison

TPLO requires considerably more soft tissue elevation around the proximal tibia than does TTA. This may explain the tendency for TTA-treated dogs to regain limb use slightly sooner than TPLO-treated dogs. There is, however, a trend amongst experienced TPLO surgeons toward reduced or no elevation of soft tissues lateral and caudal to the tibia in recent years. While both techniques are advanced procedures requiring strict attention to detail, surgeons experienced with each method tend to regard TTA a simpler technique to master. The TTA implants are typically made of titanium and have a slender profile in comparison to the more bulky TPLO implants most often made of stainless steel. Thus, medial swelling of the proximal tibial region is more usually more prominent in TPLO-treated than in TTA-treated dogs. Conversely, the prominent tibial tuberosity following TTA is, on occasion, objectionable to pet owners.

The debate of which technique is superior has not been, and likely, never will be resolved. When one considers the myriad of variables presented to the surgeon such as patient size, activity level, tibial and femoral conformation, degree of CrCLD, concurrent joint pathology, etc it seems unlikely that any single technique would be the "best" for all patients. However, each technique has potential advantages and disadvantages that when applied to specific conditions inherent to the individual patient that may make one technique a logical treatment selection over the other.

Case Selection

 Low Versus High Patellar Tendon Insertion Point. The shape of the proximal tibia, tibial tuberosity and associated patellar tendon insertion varies considerably between dogs. When performing a TTA, a low (or distal) patellar tendon insertion point upon the tibial tuberosity tends to force the surgeon to use a smaller fixation plate and there is less bony support for the cage spacer. In contrast, when performing TPLO, a low insertion point is favorable as it allows greater tibial plateau rotation while still preserving bony buttress support of the tibial tuberosity. Thus, a low insertion point may favor use of TPLO over that of TTA. A high (proximal) insertion point may favor TTA as it allows the use of a larger TTA plate and provides bony support for the cage spacer, but with TPLO would risk rotation of the tibial plateau to a point below the patellar tendon insertion (putting the tibial tuberosity at risk of fracture due to loss of bony buttress support).

 Excessive Tibial Plateau Slope/Hypoplastic Tibial Tuberosity. In cases where there is some combination of excessive tibial plateau slope and/or hypoplastic tibial tuberosity, it may not be feasible to accomplish a 90° patellar tendon-tibial plateau angle with currently available instrumentation that is limited to a 12mm wide spacer cage. Failure to adequately advance the tibial tuberosity could risk residual lameness associated with persistent postoperative stifle instability. In addition, dogs with tibial plateau angle in excess of 35° have a conformational deformity of the stifle joint that place it in a relative angle of hyperextension despite the limb itself not being in the extended position. The TTA does address this conformational deformity.

 Angular and Torsional Tibial Deformity. While both procedures are somewhat focused upon reducing cranial tibial thrust instability, loss of CrCL constraint function also puts the stifle at risk of rotational instability. Angular and/or torsional tibial deformity, when present, may contribute to this rotational instability. Because TPLO involves an osteotomy that isolates the proximal articular surface (stifle) from the distal surface (hock), it permits 3-dimensional reorientation of the tibial conformation. That is to say, tibial varus/valgus and/or internal/external torsional deformity can be treated via manipulation of the TPL osteotomy whereas it would require a separate osteotomy if required at the time of TTA. In some cases additional rotational stability can be provided by an extracapsular repair technique, such as lateral suture or Tightrope CCL.

 Patellar Luxation Requiring Tibial Tuberosity Transposition. When CrCLD is complicated by concurrent patellar luxation requiring tibial tuberosity transposition, TTA permits medial or lateral transposition of the isolated tibial crest at the same time that advancement is performed. In contrast, with TPLO, the tibial crest is attached to the remainder of the tibia such that it cannot be medially or laterally transposed independent of the sagittal plane of the tibia unless an additional osteotomy were made. This additional osteotomy adds considerable complexity to the procedure.

 Patient Size. The dimensions of the TTA cage spacer (maximum 12mm width and XX mm overall length) may limit its application in giant breeds of dogs. Methods to augment the TTA procedure in these situations have been described, but add considerable complexity to the procedure.

 The Highly Unstable Knee. Complete CrCL tear in the absence of surrounding periarticular fibrosis often causes extreme stifle instability. This is in stark contrast to the stifle that is inherently stable by surrounding fibrosis and/or significant remaining CrCL integrity. While both TPLO and TTA reduce strain on the CrCL, the advisability of leaving remaining CrCL fibers intact is debated. Because in clinical practice, a 90° patellar tendon-tibial plateau angle is seldom achieved with TTA (despite preoperative planning), residual stifle instability can be seen following TTA-treatment of the highly unstable stifle in the author's (RP) clinical experience. A similar situation can arise following TPLO and with either technique the surgeon must be prepared to supplement with a static-constraint technique when deemed necessary.

While each method has inherent advantages and disadvantages, case selection for each method is often dictated by surgeon preference and anecdotal observations. When conscientiously performed, each method can be used to consistently restore limb use of the majority CrCL-deficient dogs to a near-normal state.