An increase in median deformation/axial strain in the patellar ligament, as measured using DIC, in the CrCL-deficient stifle compared to a CrCL-intact stifle was observed in the present study. Additionally, an increase in patellar ligament strain with post-TPLO TPAs of 0° and 5° was observed compared to the post-TPLO TPAs of -5°, 10°, and 15°.
During the stance phase of gait, the cranio-caudal stability of the stifle is dependent on the CrCL; therefore, cranial tibial translation is absent in an intact stifle.23 Cranial tibial subluxation was observed in a CrCL-deficient stifle during the stance phase of gait.23 The subluxation is believed to be due to the unopposed cranial force, known as the cranial tibial thrust, being present in a CrCL-deficient stifle.23
In the present study, the median grip vertical displacement during the stance phase of walking of the CrCL-intact stifle was less than observed when the same force was applied to the CrCL-deficient stifle but no different to all other post-TPLO TPAs. This suggests that the strain measures were obtained under similar conditions.
Warzee et al. demonstrated that by performing TPLO, the joint surface should become perpendicular to the tibial functional axis during weight bearing in the stance phase of gait.16 After TPLO, the original cranial tibial thrust becomes increasingly more caudally orientated as the rotation increases and TPA decreases, ultimately resulting in a caudal tibial thurst.16 Warzee et al. investigated the effect of TPLO on tibial subluxation and tibial axial rotation.16 They showed that over-rotating the TPLO resulted in significant caudal tibial subluxation and decreased tibial axial internal rotation after TPLO.16 The axial displacement seen in our study for the transected CrCL and all the post-TPLO TPAs could be due to subluxation seen in CrCL-deficient stifles. However, the causal relationship between grip vertical displacement and femorotibial translation was not specifically investigated.
In the present study, an increase in the median patellar ligament strain was observed in the post-TPLO TPAs of the 0° and 5°. Post-TPLO TPAs of 0° and 5° eliminates the cranial tibial thrust; however, the increase in rotation would increase caudal tibial thrust translation due to over-rotation.16 Excessive rotation could heighten the risk of caudal cruciate ligament strain, tibial tuberosity fracture, fibular fracture and cause abnormal femorotibial contact mechanics.7,16,24–26 The authors believe this increase in the caudal tibial translation and over-rotation could be responsible for the increase seen in the median patellar ligament strain. The change in strain and magnitude of percent change in strain for post-TPLO TPAs of 0° and 5° supports the finding that the patellar ligament strain for these two angles are indeed increased compared to the pre-TPLO intact CrCL stifle. Therefore, we amassed enough evidence to reject our hypothesis. This finding could support the notion that tibial plateau rotation for a target TPA of 5° may be excessive. 27
A decrease in median patellar ligament strain was observed in the post-TPLO TPA 10°, which approached the strain level observed in the original intact CrCL stifle. Other studies have demonstrated that a post-TPLO TPA of 0° to 14° is sufficient for creating a dynamically stable joint and reducing the femorotibial subluxation in CrCL-deficient stifles without any adverse clinical implications.5,8,16,28,29
The authors postulate that the absence of an increase in patellar ligament strain in the under-rotated post-operative TPA of 15° could be attributed to the continued cranial tibial thrust seen in CrCL deficient stifles.23
The authors were unable to explain the rationale behind the absence of a significant increase in the outlier post-TPLO TPA − 5°. However, it is plausible to speculate on the potential impact of patellar length and patellar angle in altering the biomechanical forces acting on the patellar ligament, thereby influencing the observed outcomes.
The findings in this study contradict the findings by Drew et al.30 who demonstrated no significant change in stifle extensor mechanism load following TPLO. We speculate that this contradiction could be attributed to Drew et al.30 not focusing on isolating the strain solely to the patellar ligament when investigating the strain experienced by the ligament following TPLO. Another limitation of the ex vivo study conducted by Drew et al. was the cruciate-deficient stifle not being investigated.30
The present study model differs from previous studies investigating patellar load as strain gauges were not used to determine the strain of the patellar ligament.30,31 The rationale was that securing the gauges onto the patellar ligament would influence the integrity of the patellar ligament and only test that specific portion of the ligament. Due to its non-contact nature, DIC effectively mitigates superficial hysteresis effects arising from compliance during strain calculation from grip-to-grip displacement. 22 Unlike contact sensors like strain gauges and linear variable differential transformer sensors (LVDT), DIC does not disrupt the integrity of the test specimen.22 3D-DIC is limited to measuring surface strain. Digital volume correlation (DVC) is a technique that can measure 3D internal strain, but requires the test to be conducted in as magnetic resonance imaging (MRI) scanner. 21 The experiment did pose the limit that this could not be performed in the MRI due to the risk posed to equipment and personal due to the metal composition of the implants and the mechanical testing system.
In the present study, the 3D Green-Lagrange deformation maps led an incidental observation: there was in increase in the deformation measured in some specimens at the attachment of the patellar ligament at the tibial tuberosity compared to the rest of the patellar ligament. The increase in deformation at this location is suspected to play a role in the presence of patellar ligament desmitis pathophysiology. Mattern et al. showed radiographically that the preferentially site of patellar ligament thickening to be the distal aspect of the patellar ligament. 13 The authors propose that this observation could be explained by the alteration of the extracellular matrix in the distal third of the patellar ligament, leading to a transition towards a fibrocartilaginous composition near its insertion on the tibial tuberosity. The most fibrocartilaginous tendons are heavily loaded and permanently bent around their pulleys.32 This transition may involve changes in the arrangement and distribution of collagen fibers and the presence of specialized cells and matrix components characteristic of fibrocartilage. An increase in type II collagen has been suggested in heavily loaded tendons and ligaments.32 Further studies utilizing histological techniques (Masson’s Trichrome stain and Sirius Red)33 can provide more detailed insight into the histological changes in the patellar ligament based on anatomical locations.
There were notable limitations to our study. The model did not take all the musculoskeletal structures exerting forces on the stifle joint into consideration. Specifically, the authors did not consider the muscular deformation of the stifle extensor mechanism. The rationale was to eliminate the influence the stifle extensor mechanism lever arm, which could have played on reducing the strain in the patellar ligament as an entity. Another limitation was the lack of measurement of the cranial and caudal tibial translation that occurred at the various angles of rotation of TPLO. However, the Matlab software used to analyze the DIC images accounted for the cranial displacement of the markers as the stifle bends and, therefore all efforts were made to eliminate possible distortion to the deformation measurements obtained. The behavior of the polyethylene leader line fiber was not taken into consideration when mechanical testing was performed, the authors did however obtain the deformation values for the strain equation from the mid-substance ROI and therefore it is believed that the leader line fiber elasticity would not influence the ROI as this was fixed for each specimen regardless of the testing group. The study did not assess patellar lengths, patellar angles or clinical assessment of meniscal pathology at the various post-TPLO TPAs.