Comparison of Normograde and Retrograde Intramedullary Pinning of Feline Tibias

Introduction

Fractures of the tibia account for 15% to 20% of all fractures in small animals.^1–4 In cats, tibial fractures most commonly occur in the diaphyseal region.⁵ Surgical stabilization of these fractures is often achieved by intramedullary pinning techniques, which are relatively easy to perform, require minimal equipment, and are less expensive than many other types of repair. Normograde pin insertion is an accepted and widely used method of fixation for tibial fractures in dogs and cats.^2–4 Retrograde pin insertion is technically easier to perform and is commonly used to repair tibial fractures in clinical practice.

Studies performed in dogs indicate that retrograde insertion of intramedullary pins into the tibia involves a high risk of iatrogenic injury to structures of the stifle joint.¹⁶ Such injury is likely because the proximal bowing of the canine tibia aligns the diaphysis of the bone with the intracondylar eminence on the tibial plateau.⁶ As a result, intramedullary pins placed in a retrograde fashion often damage the cranial cruciate ligament, synovial membrane, or other stifle joint structures when the pin exits the proximal tibia.¹⁶

In fractures involving the proximal to mid-diaphyseal tibia, the risk of injury to the stifle joint can be minimized by intentionally directing the pin in a craniomedial direction during retrograde pin insertion, thus directing the pin away from the stifle joint.¹ However, in mid-diaphyseal or distal diaphyseal fractures of the tibia, the pin follows the medullary cavity and exits the proximal tibia near the intercondylar eminence on the tibial plateau, resulting in penetration of the stifle joint and interference with the femoral condyle during joint motion.⁶ Subsequently, retrograde insertion of intramedullary pins for stabilization of mid-diaphyseal tibial fracture is not recommended in dogs.¹²⁶

The effect of normograde and retrograde tibial pinning on anatomical structures within the stifle joint has not been extensively studied in cats. It has been suggested that retrograde pin placement in the cat, as in dogs, may damage structures within the stifle joint.⁵ However, the feline tibia is less angular and lacks the cranial bowing of the canine tibia, so extrapolation of data collected in dogs may not be applicable to cats.⁵⁷ The purpose of this study was to evaluate the effects of normograde and retrograde pinning of mid-diaphyseal fractures of the feline tibia on the anatomical structures of the stifle joint.

Materials and Methods

Paired pelvic limbs from five, mature, domestic shorthair feline cadavers were collected, and the distal femurs, stifle joints, and tibias were radiographed. A limited surgical approach was made to expose the medial aspect of each tibia. A transverse osteotomy was performed at the mid-diaphysis using an oscillating bone saw. The location of the osteotomy was selected by determining the midpoint between the tibial plateau and distal aspect of the medial malleolus.

The left tibia from each pair was pinned using either a normograde or retrograde pinning technique (randomly assigned). The right tibia of each pair was pinned using the opposite technique. A double-trocar Steinmann pin, measuring 60% to 70% of the tibial diaphyseal diameter, was inserted using a Jacob’s hand chuck. For normograde pin insertion, the pin was started at a point midway between the tibial tuberosity and the medial tibial condyle on the medial ridge of the tibial plateau.³ The fracture was reduced and held in reduction with bone-holding forceps as the pin was driven into the distal tibial cortex. The stifle joint was held in 90° of flexion during pin insertion. The pin was left protruding from the skin proximally. For retrograde pin insertion, the Steinmann pin entered the medullary canal at the fracture site and was driven in a craniomedial direction into and through the proximal tibial cortex, surrounding soft tissue, and skin. The fracture was reduced and held in reduction with bone-holding forceps as the pin was directed in a retrograde fashion into the distal tibial cortex.¹ The pin was left protruding from the skin proximally.

The stifle joints were exposed by careful dissection, and each pin was cut to a length of 3 to 4 mm above the tibial plateau.⁵ The joint was grossly inspected to determine where the pin penetrated the proximal tibial cortex and to identify damage to the cruciate ligaments, medial and lateral menisci, intermeniscal ligament, femoral condyles, medial joint capsule, or patellar tendon. Anatomical structures were considered damaged if they were penetrated by the pin.

After visual assessment, the joint was disarticulated by transecting the remainder of the medial and lateral joint capsule, medial and lateral collateral ligaments, and remaining soft tissue attachments. With the tibial plateau and pin exposed, digital photographs were obtained to document each pin’s exit point and its proximity to structures within the stifle joint. On the cadavers, calipers were used to measure the shortest distance (in mm) from the pin’s exit point to the insertion of the cranial cruciate ligament, the medial edge of the patellar tendon, the medial joint capsule, the medial and lateral menisci, and the cranial aspect of the tibial tuberosity.

The craniocaudal pin position was determined as described by Pardo.⁶ The distance from the cranial aspect of the tibial tuberosity to the insertion of the medial collateral ligament on the tibia was measured. The distance from the pin’s exit site to the tibial tuberosity was also measured. The craniocaudal pin position was determined by the ratio:⁶

The effect of the pinning technique (normograde or retrograde) on the distances from the pin’s exit site to anatomical structures of the stifle joint was analyzed using the paired t-test. The residuals from this model were examined using frequency histograms and normal probability plots. The data was reanalyzed using the Wilcoxon’s signed rank test if the normality assumption appeared to be substantially violated. The prevalence of penetration and direct damage to pertinent joint structures by the two pinning techniques were compared using McNemar’s test. The clinical importance of statistically significant differences was assessed using confidence intervals. A P value of ≤0.05 was considered significant for all tests.^a

Results

On visual assessments, neither normograde nor retrograde intramedullary pinning resulted in damage to the cranial cruciate ligament, medial or lateral menisci, intermeniscal ligament, femoral condyles, or medial joint capsule [Figures 1, 2]. However, during retrograde pinning, the patellar tendon was penetrated in all five cats. The pins penetrated the tendon 8.6±0.9 mm proximal to its insertion on the tibial tuberosity. Pins placed in normograde fashion exited 1.6±0.23 mm from the medial edge of the patellar tendon, but they did not penetrate or damage the tendon.

The distances from the pin’s exit site on the tibial plateau to structures of the stifle joint were measured after normograde and retrograde intramedullary pin insertions [see Table]. Pins placed in normograde fashion exited significantly closer to the cranial cruciate ligament than did pins inserted in retrograde fashion (P=0.01). Normograde pins were 2.9±0.34 mm from the cranial cruciate ligament. Retrograde pins were 4.2±0.10 mm from the cranial cruciate ligament. Pins placed in normograde fashion also exited significantly closer to the medial joint capsule than did pins inserted in retrograde fashion (P=0.02). Normograde pins were 1.9±0.11 mm from the joint capsule. Retrograde pins were 2.4±0.17 mm from the joint capsule. Pins placed in normograde fashion were not significantly closer to the medial (P=0.08) or lateral (P=0.15) menisci than those inserted in retrograde fashion.

Pins inserted in normograde fashion exited slightly more caudally on the tibial plateau as evidenced by their distance from the tibial tuberosity and measurements of craniocaudal pin position. Pins placed in normograde fashion were 3.7±0.49 mm from the tibial tuberosity (craniocaudal pin position, 0.29±0.05). Pins placed in retrograde fashion were 2.6±0.21 mm from the tibial tuberosity (craniocaudal pin position, 0.21±0.01). These differences were not statistically significant (P=0.09 and 0.12, respectively).

Discussion

It is important to avoid damaging intraarticular and periarticular structures of the stifle joint when placing intramedullary pins for internal fixation of tibial fractures. Failure to do so can result in degenerative joint disease, fracture disease, and poor limb function.⁶ Retrograde placement of intramedullary pins in the tibia is technically easy and is commonly performed in clinical practice. However, studies in dogs found that retrograde pinning of mid-diaphyseal fractures resulted in a significantly greater risk of iatrogenic damage to stifle joint structures than normograde pinning.¹⁶ Because of the proximal bowing of the canine tibia, intramedullary pins placed in retrograde fashion often exit near (or actually damage) the cranial cruciate ligament, synovial membrane, or other stifle joint structures.¹⁶

The feline tibia is less angular and lacks the cranial bowing of the canine tibia. As a result, pins inserted into the feline tibia are positioned differently relative to the stifle joint than they are in dogs. The study reported here evaluated normograde and retrograde tibial pinning in cats in order to assess damage to joint structures and to determine pin positioning relative to the cruciate ligaments, medial and lateral menisci, medial joint capsule, and patellar tendon.

In cats, intramedullary pins placed in normograde fashion exited the tibia more caudally on the tibial plateau, placing them significantly closer to the cranial cruciate ligament and the medial joint capsule than pins inserted in retrograde fashion. This is opposite from the findings in the dog where normograde pins exited more cranially on the tibial plateau and were further from the cranial cruciate ligament and synovial cavity.⁶ However, pins inserted in normograde fashion in cats did not enter the joint or damage the cruciate ligaments, joint capsule, menisci, intermeniscal ligament, patellar tendon, or femoral condyles. In comparison, the frequency of patellar tendon penetration after normograde pinning in dogs was 0% to 10%.¹⁶

Intramedullary pins placed in retrograde fashion did not damage the cruciate ligaments, joint capsule, menisci, intermeniscal ligament, or femoral condyles in cats, which was in contrast to studies performed in dogs.¹⁶ In one canine study, retrograde pins interfered with the femoral condyle in 70% of 10 cases and penetrated the joint in 40%.⁶ In another study of dogs, the pin interfered with the femoral condyle in 96% of 25 cases when it was placed in retrograde fashion, and the pin interfered in 80% of 25 cases when it was placed in retrograde fashion and was directed craniomedially during insertion.¹

However, in all the cats reported here, intramedullary pins placed in retrograde fashion (and directed craniomedially during insertion) penetrated the patellar tendon just proximal to its insertion on the tibial tuberosity. In one study of 25 dogs, 40% of pins placed in retrograde fashion penetrated the patellar tendon.¹ In another canine study, pins placed in retrograde fashion penetrated the patellar tendon in four (40%) of 10 cases.

The straighter conformation of the feline tibia allowed insertion of pins in a retrograde fashion without damage to the cruciate ligaments, joint capsule, menisci, intermeniscal ligament, or femoral condyles, as occurs in dogs. The pins exited more cranially on the tibial plateau and thus avoided the stifle joint; however, damage to the patellar tendon occurred in all five cats, even when the pin was directed craniomedially during insertion.

Conclusion

The results of this study indicated that normograde pinning of the feline tibia does not result in damage to stifle joint structures and can probably be used safely for repair of mid-diaphyseal tibial fractures in cats. Retrograde pinning, while not causing damage to intraarticular structures of the stifle joint, did penetrate the patellar tendon and must be used with caution.

Acknowledgment

The authors acknowledge Dr. Carolyn Boyle for her assistance with the statistical analyses.