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Review Article: Cranial Cruciate Disease – Is Tibial Tuberosity Advancement the Answer?

We are all very aware of the frequency of cruciate injuries in dogs, but at times it can seem quite difficult to choose the right option for the optimal management of the individual patient.

Even the basic choice between surgery and conservative management is less clear cut than it was; the tendency to leave smaller patients to ‘conservative management’ has become less popular as our understanding of the effects of an unstable stifle, meniscal injuries and subsequent joint disease has improved. However, once the decision to operate has been made, which technique offers the best chances of success? A number of studies have attempted to shed light on this, but as yet, no one method appears to prove itself as being significantly better than the alternatives.

Surgical procedures may be classified into two groups; passive and dynamic stability of the stifle. Both attempt to reduce the cranial luxation of the tibia, or tibial shear force, which generates joint injury.

fig.1 - Lateral Suture Stabilisation technique

Passive stabilisation involves the placement of autogenous (e.g. a fascial graft in ‘over the top’ surgery) or synthetic (e.g. nylon for an extracapsular lateral suture stabilisation, LSS) material to mimic the action of the cranial cruciate ligament. These act as ‘guy-ropes’ to counteract tibial shear force ( fig.1, click to enlarge).

It is now accepted by most orthopaedic surgeons that the ‘over the top’ technique is less effective in producing long term stability, in comparison with the LSS or tibial plateau levelling osteotomy (TPLO). The extracapsular LSS aims to stabilize the stifle sufficiently for the time it takes for fibrosis to develop at the stifle and allow long term stability to become established. Correct placement of the suture is critical to create constant tension in the suture throughout the normal range of motion. Eventual failure of the suture is inevitable, sometimes necessitating removal. As a consequence, the results of LSS can be inconsistent, particularly in large, lively patients.

Dynamic stabilisation of the stifle takes a different approach. Several experimental models have shown that tibial shear forces are created as a result of force vectors generated during weight bearing. In its most simple form, the slope of the tibial plateau (or tibial plateau angle, TPA) causes cranial displacement of the tibia when weight is applied to the joint. By reducing the angle of this slope, the cranial displacement – tibial shear force – can be abolished.

fig.2(a) - Closing-wedge TPLO osteotomy lines - shaded bone discarded/ used as graft.

fig. 2(b) - closing-wedge TPLO with plate fitted

fig. 3(a) - Slocum TPLO radial osteotomy

Slocum established this principle in the 1980’s with the use of a closing wedge osteotomy to reduce the TPA (fig.2).  Since then, work has progressed, leading to the triumph of marketing that is the TPLO. This procedure reduces the TPA by making a radial osteotomy in the proximal tibia, rotating the fragment until the TPA = 5° and stabilising the fragment with a bone plate (see fig.3).

For a time it was thought by many that the TPLO was the answer to the cruciate debate; however studies of long-term TPLO cases have shown that results are no better than those of the LSS. The much vaunted early

fig.3(b) - Slocum TPLO plate fitted

claims that TPLO prevents progression of degenerative joint disease have also been subsequently disproved. The complication rates following TPLO surgery are relatively high at 26-34%.

Following on from investigations into the biomechanics of human knees,  studies both in vitro and in vivo in several species led to the development of tibial tuberosity advancement (TTA, post-op view, fig.4) by Montavon in 2002.  The principal behind TTA is that weight bearing forces in the stifle are directed parallel to the patella tendon. At the normal standing angle for the canine stifle, the angle between the patella tendon and the tibial plateau (Patella tendon angle, PTA) is typically >90º. This results in the cranially directed tibial thrust (fig. 5a). When the patella tendon is perpendicular to the tibial plateau, this cranial thrust is neutralised. Therefore advancing the tibia until the patella tendon is perpendicular to the tibial plateau (PTA = 90°) eliminates the damaging tibial shear force (fig.5b). On first glance this appears to be the other side of the TPLO coin but on closer examination there are

fig.4 - Mediolateral post-op radiograph TTA

fig.5a - Force vector pre-TTA results in tibial shear force

subtleties of the procedure that suggest advantages over TPLO. TPLO alters the natural tibiofemoral articulation in a way that TTA does not, thus TTA preserves the normal load transmission across the joint and its menisci. TPLO requires an osteotomy of the weight bearing axis of the limb, the TTA does not. As a consequence, if the TPLO osteotomy is not accurately directed or if the plate is not correctly contoured, mediolateral angulation or tibial valgus/ varus may result, with subsequent pathological loading of the stifle and tarsus. This osteotomy is also of great significance in the event of implant failure when extensive and complex revision surgery will be required. The incidence of patellar tendonitis, a common complication of TPLO, is also likely to be lower following TTA. Early studies, including force plate analysis have shown TTA to be comparable with but not better than other techniques, including TPLO.

fig. 5b - advancing tibial tuberosity neutralises shear force

fig.6a - TTO osteotomy lines - shaded area of bone discarded/ used as graft.

A number of similar techniques are in existence, notably the Triple Tibial Osteotomy (TTO, see fig.6) which achieves the same result as a TPLO/TTA hybrid , but takes an additional two osteotomies to do so. Two of these cross the weight bearing axis of the tibia with the potential problems that creates as discussed above. Fracture of the tibial tuberosity is a commonly reported complication with this technique.

In summary, it remains a question of which technique to recommend. Given that LSS results can be unreliable, and the suture will fail after time, it makes sense to choose a procedure that will result in permanent stability of the stifle. Both TPLO and TTA require additional instrumentation and training however the TTA procedure is significantly quicker to perform, with all the benefits that brings to our patients.  It also appears to offer fewer major complications. Further research is still required to establish how the

fig. 6b - TTO plate in place.

biomechanics of the stifle and breed differences influence the choice of technique. In time, it may be the case that one technique has proven benefits over the others.  However, following an examination of all the evidence to date, we feel the TTA currently offers the best option for management of the majority of cranial cruciate deficient stifles.