Tibial plateau levelling osteotomy (TPLO)

Tibial plateau levelling osteotomy (TPLO) is a surgical technique that was introduced in 1993 by Slocum. It is currently one of the most widely used operations to treat cranial cruciate ligament impairment in dogs. TPLO involves a radial osteotomy carried out with specific instruments, caudal rotation of the proximal segment of the tibia and stabilisation of the tibial stumps by way of a traditional Slocum plate or an implant placed at a stable angle. This technique does not involve replacement of the cruciate ligament because, by modifying the geometry of the joint, it eliminates unwanted stresses. In particular, by correcting the slope of the tibial plateau, the technique is intended to ensure dynamic cranio-caudal stability of the stifle joint during the weight-bearing stage of walking (Video).

INTRODUCTORY NOTES ON THE BIOMECHANICS OF THE STIFLE JOINT

Cranial cruciate ligamentrupture is one of the most frequent orthopaedic pathologies in dogs. It is mainly found in medium-sized, large and giant breeds and particularly in dogs which are either overweight or very active. The aetiology is multifactorial and can involve trauma and degenerative, conformational, anatomical and autoimmune factors, each of which can contribute individually or in association to causing the disorder. The most common cause of cranial cruciate ligament rupture is not trauma, but rather a progressive degenerative disorder induced by chronic mechanical stress on the ligament. Through multiple mechanisms, weight-bearing exerts a cranial thrust on the tibia leading to progressive weakening, degeneration, partial rupture and, only later, complete rupture of the ligament. Following this, there is a progressive loss of joint stability which can lead to the development of degenerative joint disease and often to a secondary meniscal lesion. Treatment should be aimed at restoring joint stability, achieving remission of symptoms and interrupting secondary joint degenerative processes.

The model of the active biomechanics of the stifle joint developed by Slocum in 1983 showed that there is a force responsible for a constant and chronic stress on the ligament which can lead to partial rupture of the ligament. Continued stress on the ligament then leads to its complete rupture. This force is known as the cranial tibial thrust (CTT). The evolution of this biomechanical model of the stifle joint in dogs, from the traditional or static model to an active model, inevitably led to an evolution in surgical techniques. According to Slocum’s biomechanical model, the best surgical approach would re-establish a balance between the forces acting on the joint while neutralising the CTT. On this background, it is clear that surgery must be aimed at controlling the CTT rather than reconstructing the ruptured ligament or replacing its functions. In the long term, if the CTT is not neutralised, any type of reconstruction or replacement of the ruptured ligament is destined to fail since the ligament will continue to be exposed to the same forces that caused the initial rupture. According to Slocum’s theory, the entity of the CTT is directly proportional to the angle of inclination of the tibial plateau, that is, the tibial plateau angle (TPA).

Initially, this hypothesis found some consensus, but subsequent studies did not demonstrate any direct link between the TPA and cranial cruciate ligament rupture, at least as far as concerns TPA considered to be within the physiological range. In fact, the average TPA varies from 18° to 24° both in clinically healthy dogs and in those with cranial cruciate ligament rupture.  According to more recent biomechanical theories, the tibia is not subjected axially to the weight proposed by Slocum. Tepic suggests that the total force involving the stifle joint in vivo is exerted directly parallel to the tibial-patellar tendon.  According to this biomechanical model, the CTT depends, therefore, on the angle between the tibial plateau and the tibial-patellar tendon. If the tibial plateau is not perpendicular to the patellar ligament, the shear force that develops when the leg takes weight exerts an excessive stress on the cranial cruciate ligament. Building on these theories, surgical methods have been developed which, by modifying the tibial geometry, obtain dynamic joint stability without the need for prosthetic ligaments. Currently, the most widely used osteotomy techniques are TPLO and tibial tuberosity advancement. TPLO is intended to stabilise the stifle joint under weight-bearing conditions through neutralisation of the CTT, by reducing the slope of the tibial plateau. In contrast, tibial tuberosity advancement neutralises the CTT by moving the tibial crest, to which the tibial-patellar ligament is attached, forward so that it becomes perpendicular to the tibial plateau. In spite of the different biomechanical assumptions, the final results of TPLO and tibial tuberosity advancement are similar: both bring the patellar tendon perpendicular to the tibial plateau. The biomechanics of these techniques have been validated in numerous studies on cadavers which have shown the efficacy of the operations in eliminating the CTT in weight-bearing conditions. These studies have also shown that, following TPLO and tibial tuberosity advancement, the CTT is converted into a caudal-tibial thrust and, therefore, the caudal cruciate ligament becomes the main stabilising element in the stifle. 

KEY STEPS IN PRE-OPERATIVE PLANNING

Pre-operative X-ray evaluation is extremely important for the correct performance of a TPLO. The first measurement of the TPA is made on a medio-lateral X-ray, according to the principles of Slocum’s technique. The appropriate size of blade for the osteotomy is then chosen. The most appropriate size is that which, when centred at the level of the tibial intercondylar eminences, allows the caudal edge of the blade to protrude perpendicularly from the caudal cortex, as well as to have a tibial crest of appropriate thickness cranial to the osteotomy and a proximal tibial stump of sufficient size to be able to position the osteosynthesis implant correctly. After having measured the TPA and chosen the most suitable blade, the degree of rotation that should be applied to the proximal tibial fragment should be decided, based on specific conversion tables. The following distances should then be measured on a medio-lateral X-ray: i) the distance in mm between the proximal prominence of the tibial tuberosity and the circumference described by the blade on a straight line perpendicular to the tibial tuberosity; ii) the distance in mm between the proximal prominence of the tibial tuberosity and the most proximal point of the plateau intersected by the osteotomy arc. During surgery, these distances will be the landmarks for the correct positioning of the osteotome according to the pre-operative planning. 

KEY SURGICAL STEPS

Patients undergoing  TPLO must be placed on their back with the chest slightly inclined towards the side opposite to the limb involved and with the contralateral hind limb fixed to the operating table. Surgery involves a medial approach to the proximal third of the tibia including detachment of the cranial tibial muscle, the pes anserinus (retracted caudally until the medial collateral ligament can be seen) and the popliteus muscle. Arthroscopy, arthrotomy or mini-arthrotomy can all be used to examine the stifle joint during a TPLO. It is also essential to evaluate the alignment of the limb to identify any possible varus or valgus deviations or tibial torsion to be corrected during the operation. The original technique involved the use of a special jig applied on the sagittal plane at the medial side of the tibia with two screws. This device keeps the tibia in line while the proximal tibial segment is rotated and is essential to correct proximal varus or valgus tibial deviations or internal or external rotation of the foot. To avoid lesions to the soft tissues and neurovascular structures, the part of the proximal tibia where the osteotomy is being performed should be isolated with soaked gauze. The osteotomy is then performed according to the pre-operative planning.

Before finishing the incision, the scalpel and the electrocautery blade should be used to make reference marks for the reduction of the tibial stumps and stabilisation of the osteotomy. A screw the same size as those used to fix the jig should then be inserted in the plateau to rotate it. Once the proximal stump of the tibia has been reduced as planned, it can be provisionally stabilised using a temporary Kirschner pin. The joint is then permanently stabilised using an internal osteosynthesis implant according to the principles of the system chosen. The osteotomy can be stabilised using a traditional Slocum plate or a specific implant with a fixed angle (Fig. 2).

POSITION OF THE OSTEOTOMY

The osteotomy must be centred on the proximalintercondylartubercle, the landmark that ensures precise rotation. Incorrect positioning of the osteotomy will result in inadequate levelling of the tibial plateau, increasing the risk of post-operative complications such as angular and torsional deformities, and tibial crest fracture. Biomechanical studies have shown that after rotation of the proximal tibial segment, the femoral-tibial shear forces are transformed from cranial to caudal when the leg takes weight. For this reason, excessive rotation (over-rotation) can put the caudal cruciate ligament under stress. It was once thought that the optimal post-operative TPA should be between 0° and 5°. Subsequently, in vitro studies showed that the CTT is neutralised at an angle of 6.5°. Indeed, a three-dimensional computerised model indicated that rotation up to 5° would only marginally reduce the forces exerted on the ruptured cranial cruciate ligament.

Experimental studies on cadavers still have limitations due to the problems involved in reproducing the pathology as it occurs in nature and in simulating all the muscular forces which are exerted on joints in vivo. Computerised models are also inaccurate in that they do not take into consideration muscular compensation, over-simplifying joint biomechanics. Data from in vitro biomechanical analyses and from theoretical biomechanical models do not reflect clinical results, which is one of the reasons why the theoretically optimal approach to TPA has still not been objectively identified. Furthermore, the optimal TPA could vary according to a series of factors including breed and the duration of the disease. The typical dimorphism of dog breeds affects not only the average physiological values of the TPA, but also the angle of the stifle joint and, therefore, the relative slope of the tibial plateau with respect to the ground while the dog is standing still and while it is walking. It is, therefore, possible that in some dog breeds, in order to eliminate the CTT and stabilise the stifle in weight-bearing conditions, the final TPA correction should reach 5°, while for other breeds 5° or more would be sufficient. Joints affected by chronic pathologies may not require as much rotation of the proximal tibial segment as joints which have undergone acute rupture of the ligament because peri-articular fibrosis could contribute to providing functional joint stability. 

COMPLICATIONS

An analysis of the literature shows that clinical results in dogs undergoing TPLO are usually favourable. Subjective evaluation of joint function suggests rapid recovery of weight-bearing capacity with mid- to long-term results showing better joint function than that obtained using other intra- and extra-capsular methods of stabilisation. In Slocum’s original study, which included 394 dogs, results of the follow-up evaluation after more than 6 months were excellent in 73% of cases, good in 21%, and poor in 3%. Another study with a follow-up between 6 months and 4 years found that 93% of owners were reported to be satisfied. In a study of bilateral treatment in a single session (25 cases, 50 joints), most owners evaluated long-term joint function as good to excellent. Nevertheless, there are numerous reports of intra- and post-operative complications in dogs undergoing TPLO.

There are two main reasons for the high frequency of complications found: the large number of cases reported in the literature and the frequent inclusion of accounts of first experiences. The reported percentages range from 26% to 34%, including both major and minor complications (tibial crest fractures, implant collapse, inflammation of the tibial-patellar tendon, secondary meniscal damage and infections). Tibial crest fractures have been reported in 3-7% of cases but most did not require surgical revision. Too cranial a position of the osteotomy, over-rotation of the proximal tibial segment (beyond the point of insertion of the patellar tendon, the so-called “safe point”), incorrect positioning of the Kirschner wires used for the temporary stabilisation and a particularly heavy patient are all factors which can cause excessive stress on the tibial crest and, consequently, fracture. Other factors predisposing to tibial crest fractures are thermal necrosis, vascular impairment secondary to the dissection of soft tissues and excessive stress on the tibial-patellar tendon. The use of Kirschner wires and tension bands can, in selected cases, help to reduce the risk of this complication.

Patellar tendon thickening and patellar tendinosis are common complications and cause lameness in the first 2 months after TPLO. Patellar tendon thickening is a very common occurrence after TPLO, is generally asymptomatic and does not require any treatment. In contrast, patellar tendinosis is a tendon thickening associated with clinical signs such as persistent lameness, pain on flexing the stifle and on palpation of the tendon itself. A medio-lateral X-ray, possibly together with ultrasound imaging, is useful to confirm a clinical suspicion. Patellar tendinosis is more serious at the distal insertion of the tendon. Possible causes of patellar tendon thickening and patellar tendinosis include trauma to the tendon during surgery due to excessive retraction of the ligament or thermal damage caused by touching the ligament with the bi-radial blade during the osteotomy. It is, however, essential to remember that the modifications made to the biomechanics of the stifle induced by the TPLO can themselves cause changes in the tibial-patellar tendon. In fact, rotation of the proximal tibial segment decreases the jib because of the shortened distance between the insertion of the tibial-patellar ligament and the centre of rotation of the joint. Consequently, following TPLO, a stronger contraction of the quadriceps may be required to obtain the same joint extensortorque. This theory is supported by a radiographic study by Mattern and colleagues in which post-operative TPA of less than 6° were associated with clear ultrasound changes in the patellar tendon.

Infections such as septic arthritis, osteomyelitis and superficial wound infection account for 3% to 7% of complications, a higher percentage than that associated with other surgical techniques. Septic arthritis is among the most serious complications that can occur in the post-operative period. There are many reasons for this high rate of infections after TPLO. The main factors predisposing to infection are a scarcity of soft tissues covering the surgical site, long operating times, the characteristics of the plate surface, and thermal necrosis provoked by the osteotomy. In accordance with the biomechanical principles by which over-rotation (TPA less than 5°) increases the forces acting on the caudal cruciate ligament, lesions to this ligament are included among the potential complications of TPLO. Although this has been shown in studies on cadavers, there are no reports in the literature of post-operative lameness due to this type of complication, except in subjects with a post-operative TPA less than -7°.

EVOLUTION OF OSTEOARTHRITIS

There have been numerous studies evaluating the progression of osteoarthritis after TPLO. One radiographic study of 40 dogs showed a significant increase in the number of osteophytes at 6 months after TPLO. In reality, it is interesting to note that osteophytic progression was not seen in most cases (57.5%) and was increased in only two dogs. A comparison of the long-term radiographic changes seen after TPLO and those after extra-capsular stabilisation showed that while TPLO did not prevent the progression of osteoarthritis it did reduce it 3-fold with respect to the extra-capsular technique. Clinical studies evaluating the efficacy of TPLO solely on the basis of X-ray analysis should be interpreted with caution since changes to soft tissues (synovium, capsular ligaments), cartilage and menisci are not clearly seen on X-rays. Furthermore, it is interesting that the radiographic changes associated with osteoarthritis are not necessarily correlated with the clinical course and prognosis regarding the degree of limb function. 

MENISCI AND TIBIAL-FEMORAL PRESSURE DISTRIBUTION

Most cases of meniscus rupture are associated with lesions to the cranial cruciate ligament. Isolated meniscal lesions are rare. The reported incidence of torn menisci associated with cranial cruciate ligament rupture is between 50% and 90%. A recent retrospective study of 275 dogs compared four different breeds (Labrador Retriever, Rottweiler, Boxer and German Shepherd with mean TPA of  25.9°, 26.2°, 25.9° and 28.2°, respectively) and found no direct correlation between excessive inclination of the tibial plateau and meniscal lesions. Meniscal damage can be acute or degenerative and usually involves the caudal pole of the medial meniscus. The type of approach to be adopted with the meniscus of patients with cranial cruciate ligament rupture is still a subject of debate. Methods of treatment for meniscal lesions include partial or total meniscectomy and primary repair of the peripheral lesions of the meniscus. In cases in which during surgery the meniscus is seen to be still integral, meniscal release, as first proposed by Slocum, can be performed in order to prevent any secondary damage from residual CTT. Meniscal release can be performed by arthrotomy, mini-arthrotomy or arthroscopy by dividing the caudal menisco-tibial ligament of the caudal horn of the medial meniscus (caudal) or by a radial incision of the medial meniscus body, immediately caudal to the medial collateral ligament (central). Although meniscal release is often associated with TPLO, it is essential to remember that passive laxity, manifested as instability and a residual shear force which causes femoral-tibial subluxation, a potential cause of meniscal damage, has also been demonstrated in vitro following tibial tuberosity advancement surgery, albeit to a lesser extent. Meniscal release frees the caudal horn of the meniscus enabling greater mobility with the aim of preventing the meniscus being squeezed between the femoral condyle and the tibial plateau. Current concerns about meniscal release have been fuelled by the results of numerous in vitro, in vivo and ex vivo studies which have analysed the real efficacy of the procedure. The meniscus performs an important biomechanical function in the joint, increasing its stability and improving the distribution of the forces from the femoral condyles to the tibial plateau.

In humans, radial resection of the meniscal body modifies the meniscal circumferential tension and reduces the ability to distribute weight evenly. These structural and functional changes of the medial meniscus cause excessive stress to the joint with the development of focal lesions of the cartilage. The consequences of meniscus release were examined in an X-ray study which showed higher rates of arthritic progression in dogs undergoing this type of procedure. Luther and co-workers hypothesized that meniscal release changes joint kinematics and that, over the long-term, not only are the medial components of the joint involved but also the lateral and perhaps the patellar-femoral compartments. The secondary development of osteoarthritis following meniscal release is the same as that following caudal horn meniscectomy and there is no proof supporting the theory that meniscal release avoids secondary meniscal complications.

A recent retrospective study found that the incidence of meniscal lesions in joints previously subjected to meniscal release and TPLO was 3.5%. According to the authors, there was no statistically significant reduction in the incidence of post-operative meniscal lesions after meniscal release when compared to cases treated arthroscopically without meniscal release (3.3%). Although arthrotomy is traditionally considered to be an accurate method for identifying and treating meniscal lesions, data suggest that failing to identify meniscal damage before surgery plays an important role in the development of recurrent lameness caused by an underlying undiagnosed meniscal pathology. Pozzi and colleagues showed that meniscal inspection by arthrotomy is less sensitive and less specific than arthroscopy in the diagnosis of meniscal damage.

It can be concluded from the most recent literature that when the medial meniscus has been evaluated well during TPLO and the CTT is effectively neutralised, meniscal release is not recommended. In vitro biomechanical studies of joints with cranial cruciate ligament rupture subjected to TPLO show that conservative treatment of an undamaged meniscus produces better joint mechanics and, above all following tibial tuberosity advancement, results in a distribution of the femoral-tibial forces which is more like that in a non-pathological joint. The effects on femoral-tibial mechanics and on stifle kinematics during weight-bearing have been examined in vitro following both TPLO and tibial tuberosity advancement. Compared with healthy joints, joints with an impaired cranial cruciate ligament have a reduced area of contact between the femoral condyle and the tibial plateau (decreased by about 40%), caudal translation and an increase of almost 100% in the maximum pressure. During weight-bearing after TPLO, the positions of the joint surfaces of the femur and the tibia change reciprocally compared to the initial situation.

Tibial plateau levelling is followed by increased flexion of the femoral-tibial surfaces. The femoral joint surface in contact with the tibial plateau is subject to caudal translation from the centre of the femoral condyles towards the area of greatest convexity. In weight-bearing conditions, this change in position of the joint surfaces, due to a modified joint geometry, results in a reduction of approximately 12% of the area of femoral-tibial contact. There is caudal translation of the tibial plateau, an increase in the pressure load in the caudal compartment of the joint, particularly its medial part, and a reduction in the space for the menisci; all these factors could predispose to meniscal lesions, especially those of the medial meniscus. This abnormal distribution of joint forces can also contribute to degenerative processes in the cartilage.

In contrast, tibial tuberosity advancement, by maintaining a normal orientation of the femoral-tibial joint surfaces, seems to restore an almost normal femoral-tibial contact as regards the size and site of the contact area and the amount of pressure. These studies suggest that TPLO may be a risk factor for lesions to the caudal pole of an intact medial meniscus and predispose to the development of osteoarthritis after the osteotomy. In contrast, tibial tuberosity advancement causes minimal changes to the femoral-tibial pressure distribution and, according to an in vitro biomechanical study, this is associated with less risk of post-operative meniscal complications and a lesser predisposition to the development of osteoarthritis.

PROGNOSIS

TPLO is currently the method of corrective osteotomy most widely used and is recognized by many orthopaedic surgeons to be the best treatment option for cranial cruciate ligament rupture in large and giant breeds of dogs. The advantages of this technique include the geometric precision and the possibility of maintaining the original position of the distal components of the extensormechanism of the leg, patellar tendon and tibial crest without altering the biomechanics of the patellar-femoral joint. The disadvantages are the fact that TPLO is not easy to perform and it is not free of complications, such as iatrogenic deformities and tibial torsion, which could cause major changes to joint biomechanics.

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