Evaluation of the theoretical optimal angle of the tibial tunnel in transtibial anatomic posterior cruciate ligament reconstruction by computed tomography

The transtibial tunnel technique has been commonly used for posterior cruciate ligament (PCL) reconstruction, in which the graft is pulled through and fixed in the tibial tunnel. Although the subjective knee scores were improved [1], clinical outcomes were not as predictable as that in anterior cruciate ligament (ACL) reconstruction [2‐4]. Previous studies have demonstrated that the recurrent posterior laxity is one of the most common residual problems in PCL-reconstructed knees, which is related to thinning and elongation of the graft [1, 5, 6]. Inevitably, the graft will produce an acute bend around the proximal posterior tibia in transtibial tunnel technique [6, 7]. The acute bend is termed as “killer turn”. The more acute the “killer turn” for the graft emerging into the joint from a tibial tunnel, the higher risk of abrasion against the anterior ‘lip’ of the internal tibial tunnel aperture, thus leading to the enlargement of the tunnel inlet and attenuation of the graft [8].

Furthermore, previous studies had shown that the fixation strength of interference screw was significantly related to local thickness of cortical bone [9, 10]. And they recommended that fixation site should be placed away from the joint line. Accordingly, numerous surgeons tend to increase the tibial tunnel angle to achieve a smaller “killer turn” effect and stronger graft fixation. Whereas, several investigators had found in animal studies that the graft healing in cancellous-filled femoral tunnel was superior to that in marrow-dominated tibial tunnel [11‐13]. They concluded that the healing of tendon-to-bone was highly correlated with peri-graft bone mass and connectivity, especially the cancellous bone architecture at the graft site. Because of the tibial marrow cavity, the peripheral cancellous bone mass of PCL graft would decrease with the increases in tibial tunnel angle. Thus leading to an adverse impact on graft healing. Therefore, there should be a compromise between reducing the “killer turn” angulation, increasing fixation strength and decreasing quality of bony tunnel. We speculated that the optimal position of the graft within the tibial tunnel was the proximal vertex of the tibial marrow cavity, which not only improve the “killer turn” angulation and fixation strength but also produce the satisfactory healing of tendon-to-bone. The theoretical optimal angle (TOA) is formed between the tibial tunnel through the proximal vertex of the tibial marrow cavity and tibial plateau.

The purpose of this study was to evaluate TOA of the tibial tunnel in transtibial anatomic PCL reconstruction through computed tomography (CT) measurements on the tibial anatomic characteristics.

In total, 408 knees were included in the present study (based on inclusion and exclusion criteria). There were 230 left knees and 178 right knees. The number was slightly higher in male patients (225) than that in female patients (183). The average age of patients was 38.3 ± 14.1 years (18–60 years in males, 18–60 years in females).

Based on sex, Table 1 showed the tibial anatomic parameters. The average value of TOA and L3 were 35.4 ± 7.9 ° and 26.8 ± 11.4 mm, respectively. There were no significant differences between sex groups (P > 0.05). The value of L1 and L2 were significant higher in males than that in females (L1, P = 0.002; L2, P = 0.046).

Regarding age, the relative results were shown in Table 2. We stratified the cases into three groups: the young (18–30 years old, n = 153), the middle-aged (31–45 years old, n = 104), the elderly (46–60 years old, n = 151). In total, the value of L1, TOA, L2 and L3 were significant higher in the middle-aged than the young (P = 0.02, P = 0.001, P = 0.038, P = 0.032, respectively). No significant differences were found in the value of measurements between the elderly and other age groups (P > 0.05).

With regard to height, we stratified the cases into three groups: I- < 1.66 m (n = 161), II - 1.66 to 1.75 m (n = 207) and III - > 1.75 m (n = 40), as shown in Table 3. The mean value of L1 was significant lower in group I than that in group II and group III (I v II, P = 0.015, I v III, P = 0.026). Similarly, the average value of L2 was also significant lower in group I than that in group II and group III (I v II, P = 0.026, I v III, P = 0.006). But there were no differences between group II and group III (P > 0.05). Interestingly, the average value of TOA and L3 showed no significant differences among three height groups (P > 0.05).

Inter- and intra-observer reliability of measurements were analyzed by ICCs. Table 4 shows the values were range from 0.641 to 0.909, indicating good reliability.

Present study evaluated the theoretical optimal angle (TOA) of the tibial tunnel on CT sagittal plane in transtibial anatomic PCL reconstruction. And the corresponding distance of TOA from anterior tunnel aperture to anterior end of tibial plateau (L2) and tibial tuberosity (L3) were also measured.

Previous studies have shown that clinical outcomes of PCL reconstruction were not so desirable compared with ACL reconstruction [2‐4]. Many researchers believed that the “killer turn” for the graft emerging into the joint from a tibial tunnel could result in graft abrasion and tunnel inlet enlargement, which were critical causes to recurrent laxity after transtibial PCL reconstruction [6‐8]. Meanwhile, animal studies suggested that the graft fixation strength with interference screw within the tibial tunnel was closely related to local thickness of cortical bone [9, 10, 15]. Therefore, surgeons tend to increase the tunnel angle to relieve the “killer turn” effect and increase the fixation strength. However, studies on rabbit models had shown that the peri-graft cancellous bone mass significantly influenced the healing of tendon-to-bone [11, 12]. For human tibias, the farther away from the knee joint line, the less the cancellous bone mass in proximal half. To our knowledge, this study firstly proposed the theoretical optimal angle of the tibial tunnel in transtibial anatomic PCL reconstruction that taking the “killer turn” effect, the graft fixation strength with interference screw and the healing of tendon-to-bone into account.

This study demonstrated that the average value of TOA, L2 and L3 were 35.4 ± 7.9 °, 47.0 ± 11.1 mm and 26.8 ± 11.4 mm, respectively. In other words, the graft will be located in marrow-dominated tunnel rather than cancellous-filled tunnel if a value of tunnel is higher than the corresponding limit. Thus, we speculated that there would be an adverse impact on the healing of tendon to bone according to the previous rabbit studies though they did not research tunnel angle [11, 12]. There is value in defining the optimal distances from anterior end of tibial plateau (L2) and tibial tuberosity (L3) to anterior tunnel aperture, but L2 is hard to clinically use due to difficulty in accuracy of palpation of tibial plateau. Actually, L3 could be positioned effortlessly on most people. Interestingly, L2 was influenced by sex, age and height of a different degree in the present study, while TOA and L3 were only influenced by age. Thus, TOA and L3 were relative constant references. X-ray measurement would be more accurate than direct vision or palpation to determine the value of TOA and L3 during PCL reconstruction. As a result, theoretical TOA (calculated by choice of ideal tunnel placement on sagittal CT images on this study) for transtibial anatomic PCL reconstruction was 35.4 ± 7.9 °. There was no effect of sex and height on TOA but it might be influenced by age.

In summary, through the computed tomography (CT) measurements on the tibial anatomic characteristics, we found that the theoretical optimal angle of the tibial tunnel relative to tibial plateau and the corresponding distance from anterior tunnel aperture to the tibial tuberosity respectively were 35.4 ± 7.9 ° and 26.8 ± 11.4 mm in transtibial anatomic PCL reconstruction. However, further clinical validation is needed.