Safe corridors for external skeletal fixator pin placement in feline long bones

Abstract

Objectives

External skeletal fixation is an established technique in cats for biological fixation of long bone fractures, stabilisation of the joints, and treatment of shearing injuries and angular deformities. As appropriate and accurate pin insertion is imperative for a successful outcome, knowledge of topographic anatomy and areas that are safe (safe corridors) for pin placement is integral to successful surgery. At present, however, safe corridors have not been determined fully in feline orthopaedics, with surgeons having to rely on knowledge based on canine orthopaedics. This study was performed to determine safe corridors for pin placement in feline long bones.

Methods

The limbs of six feline cadavers were frozen. Only limbs with no history of orthopaedic conditions were used. Transverse sections through the limbs were examined, and anatomical structures were determined in relation to the bone. These structures were compared with those of the contralateral limbs, which were dissected for topographic assessment. Safe corridors were defined as topographic areas where no vital structures, muscles or joints were present.

Results

Examination of the humerus revealed safe corridors at its proximal craniolateral aspect and on the medial and lateral humeral condyles. Safe corridors of the antebrachium were identified on the lateral aspect of the olecranon, the distal two-thirds of the medial antebrachium and the distal third of the lateral antebrachium. Safe corridors in the femur consisted of a small area lateral to and just below the major trochanter, and on the medial and lateral femoral condyles. Evaluation of the tibia revealed safe corridors on the medial aspect of the entire tibia, the cranial aspect of the proximal tibia on the tibial crest and the area just proximal to the lateral malleolus.

Conclusions and relevance

Safe corridors for pin placement during external skeletal fixation in feline limbs differed from those in canine limbs. Knowledge of canine anatomy may be inapplicable to pin placement in feline limbs undergoing external skeletal fixation.

Introduction

External skeletal fixation is frequently used in feline orthopaedics to treat conditions such as fractures, joint luxation and shearing injuries, as well as for the correction of angular deformities.^1–5 External skeletal fixation is especially advantageous for the biological fixation of fractures, as it requires minimal handling of the fracture site and soft tissues, which can result in rapid healing. This is an important consideration in cats, in which fractures are frequently complex or open. ⁶ Application of external skeletal fixation in cats requires the transverse placement of percutaneous pins into bone, followed by extracutaneous fixation to connect these pins.

Successful external skeletal fixation depends on many factors, such as the surgeon’s experience and knowledge of the topographic anatomy of the affected area. Optimal pin placement requires a thorough knowledge of cross-sectional limb anatomy, to avoid inadvertent neurovascular injury and intra-articular pin penetration. ⁷ Although external skeletal fixation has been performed in many species and in many limb regions, studies to date have not fully determined the cross-sectional anatomy, or identified the safe anatomical corridors in the forelimbs and hindlimbs of cats. External skeletal fixation has been used to treat fractures in the long bones of cat limbs,^8,9 but pin placement often relies on the knowledge of safe corridors in dogs. Only one study to date has described limb corridors in cats, with that study focusing on the humerus. ¹⁰ More accurate placement of external fixation pins in cats depends on exact knowledge of their limb anatomy. The present study evaluated the cross-sectional anatomy of the feline limbs to more accurately determine the safe corridors for pin placement during external fixations.

Materials and methods

Materials

The cadavers of six adult cats (three males and three females) euthanased for reasons unrelated to this study were provided by their owners, who also provided informed consent for participation in the study. None of these cats had a history of limb disease or deformity. The forelimbs were disarticulated at the shoulder joint and hindlimbs at the hip joint. One limb of each pair was skinned and used for topographic dissection, whereas the other, contralateral limb was stored at −14°C for subsequent cross-sectioning. Freezing of the disarticulated limbs was carried out with the limbs suspended on a hook to imitate the standing position and to prevent any pressure deformities.

Methods

The frozen limbs were transected at four section levels, as described below, using a hand saw. Each section was photographically recorded using a CANON IXUS 185 digital camera. One forelimb and one hindlimb of each animal were topographically dissected to determine the locations of vital structures, such as blood vessels, nerves, muscles and tendons, in relation to the corresponding long bones and joints along the whole extent of the limb.

Brachium

The first section of the brachium was located 2 cm below the origin of the palpable greater tubercle of the humerus. This was followed by three additional equal sections, with the most distal section located 2 cm proximal to the palpable medial epicondyle of the humerus (Figure 1).

Figure 1

Transverse sections of the right humerus. (a) Level A: 1 = m. triceps brachii – caput longum; 2 = m. triceps brachii – caput laterale; 3 = m. triceps brachiicaput accessorium; 4 = m. triceps brachiicaput mediale; 5 = m. brachialis; 6 = m. cleidobrachialis; 7 = m. deltoideuspars acromialis; 8 = m. biceps brachii; 9 = m. coracobrachailis; 10 = a. brachialis; 11 = n. medianus; 12 = n. ulnaris; 13 = v. brachialis; 14 = n. radialis. (b) Level B: 1 = m. triceps brachii – caput longum; 2 = m. triceps brachii – caput laterale; 3 = m. triceps brachiicaput accessorium; 4 = m. triceps brachiicaput mediale; 5 = m. brachialis; 6 = m. cleidobrachialis; 7 = m. deltoideuspars acromialis; 8 = m. biceps brachii; 9 = m. coracobrachailis; 10 = a. brachialis; 11 = v. brachialis; 12 = n. medianus; 13 = n. ulnaris; 14 = n. radialis. (c) Level C: 1 = m. triceps brachii – caput longum; 2 = m. triceps brachii – caput laterale; 3 = m. triceps brachiicaput accessorium; 4 = m. triceps brachii – caput mediale; 5 = m. brachialis; 6 = m. cleidobrachialis; 7 = m. deltoideuspars acromialis; 8 = m. biceps brachii; 9 = m. coracobrachailis; 10 = a. brachialis; 11 = v. brachialis; 12 = n. medianus; 13 = n. ulnaris; 14 = n. radialis. (d) Level D: 1 = m. triceps brachii – caput longum; 2 = m. triceps brachii – caput laterale; 3 = m. triceps brachiicaput accessorium; 4 = m. triceps brachiicaput mediale; 5 = m. brachialis; 6 = m. brachioradialis; 7 = m. biceps brachii; 8 = m. pectoralis descendens; 9 = a. brachialis; 10 = n. medianus; 11 = n. ulnaris; 12 = v. cephalica; 13 = n. radialis. (e) Humerus with the location of individual sections and locations of the safe corridors (green colour). H = humerus; Cr = cranial; Ca = caudal; L = lateral; M = medial

Antebrachium

The first section of the antebrachium was located 2 cm distal to the palpable radial tuberosity on the medial aspect of the proximal radius. This was followed by three additional sections at equal distances, with the most distal section located 2 cm proximal to the palpable styloid process of the radius (Figure 2).

Figure 2

Transverse sections of the right antebrachium. (a) Level A: 1 = m. brachioradialis; 2 = m. extensor carpi radialis; 3 = m. supinator; 4 = m. extensor digitorum communis; 5 = m. extensor digitorum lateralis; 6 = m. abductor pollicis longus; 7 = m. extensor pollicis et digiti secondi; 8 = m. extensor carpi ulnaris; 9 = m. flexor carpi ulnaris; 10 = m. flexor digitorum profundus – caputulnare; 11 = m. flexor digitorum profundus – caput humerale; 12 = m. flexor digitorum profundus – caput radiale; 13 = m. pronator teres; 14 = m. flexor digitorum superficialis; 15 = m. flexor carpi radialis; 16 = v. cephalica; 17 = n. radialis; 18 = a. antebrachialis cranialis superficialis; 19 = a. brachialis; 20 = n. ulnaris; 21 = n. medianus. (b) Level B: 1 = m. brachioradialis; 2 = m. extensor carpi radialis; 3 = m. supinator; 4 = m. extensor digitorum communis; 5 = m. extensor digitorum lateralis; 6 = m. abductor pollicis longus; 7 = m. extensor pollicis et digiti secondi; 8 = m. extensor carpi ulnaris; 9 = m. flexor carpi ulnaris; 10 = m. flexor digitorum profundus – caput ulnare; 11 = m. flexor digitorum profundus – caput humerale; 12 = m. flexor digitorum profundus – caput radiale; 13 = m. pronator teres; 14 = m. flexor digitorum superficialis; 15 = m. flexor carpi radialis; 16 = v. cephalica; 17 = n. radialis; 18 = a. antebrachialis cranialis superficialis; 19 = a. interossea cranialis et caudalis; 20 = n. ulnaris; 21 = n. medianus. (c) Level C: 1 = m. brachioradialis; 2 = m. extensor carpi radialis; 3 = m. supinator; 4 = m. extensor digitorum communis; 5 = m. extensor digitorum lateralis; 6 = m. abductor pollicis longus; 7 = m. extensor pollicis et digiti secondi; 8 = m. extensor carpi ulnaris; 9 = m. flexor carpi ulnaris; 10 = m. flexor digitorum profundus – caput ulnare; 11 = m. flexor digitorum profundus – caput humerale; 12 = m. flexor digitorum profundus – caput radiale; 13 = m. flexor digitorum superficialis; 14 = m. flexor carpi radialis; 15 = n. radialis; 16 = v. cephalica; 17 = a. antebrachialis cranialis superficialis; 18 = a. interossea cranialis et caudalis; 19 = n. ulnaris; 20 = n. medianus. (d) Level D: 1 = m. brachioradialis; 2 = m. extensor carpi radialis; 3 = m. extensor digitorum communis; 4 = m. extensor digitorum lateralis; 5 = m. extensor carpi ulnaris; 6 = m. pronator quadratus; 7 = m. flexor carpi ulnaris; 8 = m. flexor digitorum profundus – caput ulnare; 9 = m. flexor digitorum profundus – caput humerale; 10 = m. flexor digitorum profundus – caput radiale; 11 = m. flexor carpi ulnaris; 12 = n. radialis; 13 = v. cephalica; 14 = a. antebrachialis cranialis superficialis; 15 = a. interossea cranialis et caudalis; 16 = n. ulnaris; 17 = n. medianus; 18 = m. flexor digitorum superficialis. (e) Radius with the location of individual sections and locations of the safe corridors (green colour). R = radius; U = ulna; Cr = cranial; Ca = caudal; L = lateral; M = medial

Thigh

The first section of the femur was located 2 cm distal to the base of the palpable greater trochanter, followed by three additional sections at equal distances. The most distal section was located 2 cm proximal to the palpable lateral epicondyle of the femur (Figure 3).

Figure 3

Transverse sections of the right thigh. (a) Level A: 1 = m. quadriceps femoris – m. vastus lateralis; 2 = m. quadriceps femoris – m. rectus femoris; 3 = m. quadriceps femoris – m. vastus intermedius; 4 = m. quadriceps femoris – m. vastus medialis; 5 = m. sartorius – pars cranialis; 6 = m. sartorius – pars caudalis; 7 = m. pectineus; 8 = m. adductor longus; 9 = m. adductor brevis; 10 = m. adductor magnus; 11 = m. gracilis; 12 = m. semimembranosus; 13 = m. semitendinosus; 14 = m. biceps femoris; 15 = n. ischiadicus; 16 = a. femoralis; 17 = n. saphenus. (b) Level B: 1 = m. quadriceps femoris – m. vastus lateralis; 2 = m. quadriceps femoris – m. rectus femoris; 3 = m. quadriceps femoris – m. vastus intermedius; 4 = m. quadriceps femoris – m. vastus medialis; 5 = m. sartorius – pars cranialis; 6 = m. sartorius – pars caudalis; 7 = m. pectineus; 8 = m. adductor longus; 9 = m. adductor brevis; 10 = m. adductor magnus; 11 = m. gracilis; 12 = m. semimembranosus; 13 = m. semitendinosus; 14 = m. biceps femoris; 15 = n. ischiadicus; 16 = a. femoralis; 17 = n. saphenus. (c) Level C: 1 = m. quadriceps femoris – m. vastus lateralis; 2 = m. quadriceps femoris – m. rectus femoris; 3 = m. quadriceps femoris – m. vastus intermedius; 4 = m. quadriceps femoris – m. vastus medialis; 5 = m. sartorius – pars cranialis; 6 = m. sartorius – pars caudalis; 7 = m. pectineus; 8 = m. adductor longus; 9 = m. adductor brevis; 10 = m. adductor magnus; 11 = m. gracilis; 12 = m. semimembranosus; 13 = m. semitendinosus; 14 = m. biceps femoris; 15 = n. ischiadicus; 16 = n. saphenus; 17 = a. femoralis. (d) Level D: 1 = m. quadriceps femoris – m. vastus lateralis; 2 = m. quadriceps femoris – m. rectus femoris; 3 = m. quadriceps femoris – m. vastus intermedius; 4 = m. quadriceps femoris – m. vastus medialis; 5 = m. sartorius – pars cranialis; 6 = m. sartorius – pars caudalis; 7 = m. pectineus; 8 = m. adductor magnus; 9 = m. semimembranosus; 10 = m. gracilis; 11 = m. semitendinosus; 12 = m. biceps femoris; 13 = a. femoralis; 14 = n. saphenus; 15 = n. ischiadicus. (e) Femur with the location of individual sections and locations of the safe corridors (green colour). F = femur; Cr = cranial; Ca = caudal; L = lateral; M = medial

Crus

The most proximal section of the tibia was located at the level of the distal extent of the tibial crest. This was followed by three equal sections, with the most distal being located 2 cm above the medial malleolus of the tibia (Figure 4).

Figure 4

Transverse sections of the right crus. (a) Level A: 1 = m. tibialis cranialis; 2 = m. extensor digitorum longus; 3 = m. extensor digitorum lateralis; 4 = m. peroneus longus; 5 = m. peroneus tertius; 6 = m. flexor hallucis longus; 7 = m. flexor digitorum longus; 8 = m. tibialis caudalis; 9 = m. popliteus; 10 = m. soleus; 11 = m. gastrocnemius – caput mediale; 12 = m. gastrocnemius – caput laterale; 13 = m. flexor digitorum superficialis; 14 = m. biceps femoris; 15 = a. et v. tibialis cranialis; 16 = n. peroneus; 17 = n. tibialis; 18 = a. saphena; 19 = v. saphena lateralis; 20 = v. saphena medialis. (b) Level B: 1 = m. tibialis cranialis; 2 = m. extensor digitorum longus; 3 = m. extensor digitorum lateralis; 4 = m. peroneus longus; 5 = m. peroneus tertius; 6 = m. flexor hallucis longus; 7 = m. flexor digitorum longus; 8 = m. popliteus; 9 = m. soleus; 10 = m. gastrocnemius – caput mediale; 11 = m. gastrocnemius – caput laterale; 12 = m. flexor digitorum superficialis; 13 = a. et v. tibialis cranialis; 14 = n. peroneus; 15 = n. tibialis; 16 = a. saphena; 17 = v. saphena lateralis; 18 = v. saphena medialis. (c) Level C: 1 = m. tibialis cranialis; 2 = m. extensor digitorum longus; 3 = m. peroneus longus; 4 = m. extensor digitorum lateralis; 5 = m. peroneus tertius; 6 = m. peroneus brevis; 7 = m. flexor digitorum longus; 8 = m. flexor hallucis longus; 9 = m. soleus; 10 = m. gastrocnemius; 11 = a. et v. tibialis cranialis; 12 = n. peroneus; 13 = n. tibialis; 14 = a. saphena; 15 = v. saphena lateralis; 16 = v. saphena medialis. (d) Level D: 1 = m. tibialis cranialis; 2 = m. extensor digitorum longus; 3 = m. peroneus longus; 4 = m. peroneus tertius; 5 = m. peroneus brevis; 6 = m. flexor digitorum profundus; 7 = tendo calcaneus communis; 8 = a. et v. tibialis cranialis; 9 = n. peroneus; 10 = n. tibialis; 11= a. saphena; 12 = v. saphena lateralis; 13 = v. saphena medialis. (e) Tibia with the location of individual sections and locations of the safe corridors (green colour). T = tibia; F = fibula; Cr = cranial; Ca = caudal; L = lateral; M = medial

Each section was compared with the topography of individual structures in the dissected, contralateral limb. Three types of corridors for pin insertion were identified, based on the presence or absence of vital structures, as described previously ¹¹ with some modifications.

Safe corridor

A safe corridor was defined as an area without any vital structures, such as neurovascular bundles, large muscles and their tendons. An ideal safe corridor was defined as an area of bone palpable immediately below the skin.

Hazardous corridor

A hazardous corridor was defined as an area without neurovascular bundles, but that could contain musculotendinous structures and/or a moderate amount of subcutaneous tissues.

Unsafe corridor

An unsafe corridor was defined as an area containing bundles of neurovascular tissue and/or large musculotendinous structures.

Results

Knowledge of the locations of safe, hazardous and unsafe corridors on the brachium, antebrachium, thigh and crus is important for fracture repair in cats using external skeletal fixation pins. At least one safe corridor for pin insertion was identified in each part of the limbs.

Brachium (Figure 1)

A safe corridor of the brachium was detected at the proximal aspect of the humerus craniolaterally. This corridor was identified as a palpable bone just below the skin at the site of attachment of the deltoid muscle. It extended 3 cm distally from the base of the greater tubercle to 3 cm distally. A second safe corridor was identified as a circular area about 1 cm in diameter around the lateral and medial epicondyles of the humerus. The distal third of the lateral aspect of the humerus was identified as an unsafe corridor due to the presence of the radial nerve and its branches. The entire extent of the caudal aspect of the humerus was also identified as an unsafe corridor due to the presence of the triceps muscle mass and neurovascular bundles located proximally and distally, represented by the caudal circumflex artery and vein and axillary nerve, and the median nerve and deep brachial artery, respectively. The entire medial aspect of the brachium excluding the centre of the condyle was considered an unsafe corridor due to the presence of large neurovascular bundles, represented by the brachial artery and vein, and the ulnar, median and musculocutaneous nerves and their branches. The median nerve was found to pass through a supratrochlear foramen located medially approximately 1 cm above the medial epicondyle of the humerus crossing from the caudal to the cranial aspect. The cranial aspect of the brachium was identified as a hazardous corridor due to the presence of the biceps brachii muscle, under which the humerus was easily palpated, with no neurovascular structures detected.

Antebrachium (Figure 2)

The only safe corridor identified on the antebrachium was the distal third of its medial side, on which the medial ridge of the radius could be easily palpated just below the skin. The area of the proximal ulna/olecranon, including both its medial and lateral aspects, was considered a hazardous region due to attachments and origins of the brachial and antebrachial muscles. Furthermore, the caudal ridge of the olecranon was accompanied by the ulnar nerve in the groove between the extensor and flexor musculature, making this area an unsafe corridor. Another hazardous corridor was located on the medial and lateral aspects of the distal third of the antebrachium, where only musculotendinous structures were present and both the radius and ulna could be palpated between the tendons. The entire extent of the cranial (position of the radius) and caudal (position of the ulna) aspects of the antebrachium was considered an unsafe corridor due to the presence of large flexor (caudally) and extensor (cranially) musculatures accompanied by neurovascular bundles of the median and musculocutaneous nerves and the median artery and vein caudally.

Thigh (Figure 3)

The only safe corridors identified were a small area at the base of the major trochanter and the medial and lateral femoral condyles, locations at which the femur could be easily palpated. An area extending along the lateral aspect of the femur from the level of the base of the greater trochanter to the lateral femoral epicondyle was identified as a hazardous corridor, with the femur being palpable between the lateral vastus muscle and the biceps femoris. All other areas of the femur were unsafe corridors due to the presence of large neurovascular bundles medially (ie, the saphenous nerve and the femoral artery and vein) and caudolaterally (ie, the tibial and peroneal nerves and their branches, and the caudal femoral circumflex artery and vein), and large muscular layers cranially and caudally.

Crus (Figure 4)

A safe corridor was identified along the entire extent of the medial tibia, with the bone being palpable just below the skin and without any muscles or neurovascular tissue. Other safe corridors included the cranial aspect of the proximal tibia along the tibial tuberosity and tibial crest, and an area at the level just proximal of the lateral malleolus. The area of the lateral proximal tibia just below the level of the stifle joint was a hazardous region, apart from an area of the nerve location considered as unsafe, where a deep branch of the peroneal nerve, 2 cm below the level of tibial joint surface, was identified. This nerve was found to continue craniolaterally along the tibia, making the entire extent of the craniolateral tibia an unsafe corridor. Caudally, the tibial region was considered an unsafe corridor owing to the presence of large musculature with large neurovascular bundles.

Discussion

External skeletal fixation is used in small animal orthopaedics primarily for biological fixation of different types of fractures, stabilisation of the joints following luxations or subluxations, and correction of angular deformities of the limbs.^2,5,8,9,12 The selection and application of an appropriate type of external skeletal fixation depends on many factors, including the surgeon’s experience and knowledge of the topography of the selected area. Inappropriate insertion of an external skeletal fixation pin can lead to various complications, including pin loosening, pin tract infection, entrapment or damage of neurovascular structures, pain and neuropraxia.^13,14 Despite the widespread use of external skeletal fixation in cats, safe corridors in limbs have not been accurately described. However, some aspects of limb topography differ significantly in cats and dogs. Two safe corridors were observed in the feline brachium. Use of the craniolateral side of the humerus is possible owing to the absence of large muscle groups, blood vessels and nerves. This area can be identifiable by palpation of the tuberculum majus humeri and distally the deltoid tuberosity.

The lateral epicondyle of humerus, identifiable by palpation, can be safely used for pin insertion during external skeletal fixation. Similarly, the medial epicondyle of the humerus can be used for pin insertion in hybrid fixation, with great attention to proper placement as median nerve is located in the supracondylar foramen just proximal to the medial epicondyle. These findings agree with those of a previous study, ¹⁰ which reported two corridors in the feline humerus, one on the craniolateral side and the other on the lateral epicondyle.

Although the safe corridors on the feline brachium were similar to their canine counterparts proximally, ¹⁵ the distal part of the feline humerus contains the supratrochlear foramen with its median nerve which is not present in dogs, making this area an unsafe corridor in cats. A hazardous corridor is present on the caudal side of the feline humerus due to the presence of the musculus triceps brachii, the most important extensor of the elbow joint. It is not appropriate to insert pins through a thick layer of muscle due to the risks of loosening, inflammatory reactions of soft tissues and pain. ¹⁴ The radial nerve runs between the caput mediale and caput accessorium musculi tricipitis brachii, with any iatrogenic damage resulting in significant morbidity.

An unsafe corridor is located on the medial side of the brachium, except for the distal area of the medial epicondyle. The arteria and vena brachialis, along with the ulnar nerve and median nerve, run at all levels from A to D. The median nerve is located along musculus biceps brachii and, in the distal part of the humerus, runs together with the brachial artery and musculus epitrochleoanconeus through the supracondylar foramen. The ulnar nerve runs together with the median nerve distally but does not pass through the supracondylar foramen. The ulnar nerve separates from the median nerve in level D and continues to the olecranon ulnae. An unsafe corridor was also found in the distal part of the humerus at levels C and D. The radial nerve runs from the caudal to the lateral side of the humerus. This unsafe corridor is the same for both the dog and cat.

In cats, however, the distal part of the brachium, except for the epicondyles, represents an unsafe corridor owing to the supracondylar foramen, ¹⁰ whereas, in dogs, the distal part of the humerus can be used for pin insertion. ¹⁵

The medial side of the radius represents a safe corridor for pin insertion, where the radius is easily identifiable below the skin by palpation. Antebrachial muscles interfere with the medial aspect of the ulna except its tuber olecrani, making it an unsafe corridor. It is therefore necessary to insert the pin in the medial direction and to avoid craniomedial or caudomedial movement due to the presence of the median nerve medially and radial nerve laterally. The placement of unilateral or bilateral external skeletal fixation is possible at this site, but the lateral aspect of the antebrachium remains a hazardous corridor owing to the position of the musculus extensor digitorum lateralis, the musculus extensor carpi ulnaris and the musculus flexor carpi ulnaris – muscles responsible for extension and flexion in carpal and digital joints. Owing to the thin muscle layer, it is possible to insert pins in the distal part of the lateral side of the antebrachium into the radius. The musculus extensor carpi radialis, musculus extensor digitorum communis and musculus supinator are located on the cranial side of the antebrachium. These muscles, as well as the radial nerve, may be damaged when pins are inserted into the cranial part of the radius. The antebrachial artery superficialis cranialis and the vena cephalica of the antebrachium run together with the radial nerve on the cranial aspect of the antebrachium. Pin insertion can be performed despite this being a hazardous area as the cephalic vein can easily be identified subcutaneously.

The musculus flexor digitorum superficialis, musculus flexor digitorum profundus and ulnar nerve are located on the caudal side of the antebrachium. The median nerve runs from the medial to the caudomedial side of the proximal third of the antebrachium. The position of these muscles and nerves makes pin placement on the caudal side of the antebrachium inappropriate. Overall, corridors on the feline antebrachium are distributed similar to corridors on the canine antebrachium. ¹⁵

Safe corridors on the feline femur were located at the levels of the medial and lateral femoral condyles and a small area laterally at the base of the major trochanter. The lateral aspect of the femur can be considered for pin insertion despite it being a hazardous region. Pins must be inserted laterally into the intermuscular space between the vastus lateralis and biceps femoris. Pin insertion is possible immediately distal to the greater trochanter of the femur and at the level of the lateral epicondyle. Use of different configurations of external skeletal fixation on the thigh have been reported. However, all were placed only laterally because of the position near the body wall. Our findings confirm the previous description of pin placement for external skeletal fixation, ¹⁶ which was based on canine anatomy. ¹¹ The musculus vastus medialis, musculus gracilis, musculus sartorius, musculus pectineus and all adductors of the pelvic limb are located on the medial side of the femur. The femoral artery, which supplies blood to the musculus quadriceps femoris, runs near the musculus vastus medialis. Damage to the femoral artery during surgery results in severe haemorrhage. The saphenous nerve passes together with the femoral artery in this area. The femoral artery and vein and the saphenous nerve are located at all levels on the medial side. Damage to these structures is detrimental to the function of the limb. The musculus biceps femoris, musculus semitendinosus and musculus semimembranosus are located on the caudal side of the femur. The ischiadic nerve runs between the musculus adductor magnus and above mentioned muscles. The presence of the ischiadic nerve at all levels on the caudal side absolutely eliminates the possibility of a caudal approach. The ischiadic nerve innervates muscles located on the caudal side of the femur and all muscles of the crus and pes. All levels of the medial and caudal sides of the femur are therefore defined as unsafe corridors.

Safe corridors are located medially on the tibia, cranially on the tibial crest and tuberosity, and on the lateral malleolus. Cranial and medial approaches can be used owing to the subcutaneous position of the tibia and the absence of large muscle groups, blood vessels and nerves. The caudal side of the tibia is an unsafe corridor owing to the presence of the tibial nerve, vena saphena lateralis and large muscles, including the musculus gastrocnemius, musculus flexor digitorum superficialis, musculus biceps femoris and musculus flexor digitorum profundus. An unsafe corridor is located on the lateral side of the tibia with the exception of the distal part corresponding to the lateral malleolus of the tibia. The musculus extensor digitorum longus, musculus extensor digitorum lateralis, musculus peroneus longus and musculus peroneus tertius are located in all levels laterally. Corridors of the tibia as described in this study are similar to those described in dogs. ¹¹

Conclusions

Despite anatomical similarities between dogs and cats, careful evaluation is required prior to pin insertion during external skeletal fixation in cats. The findings of this anatomical study should therefore be considered, especially for pin insertion in areas of the brachium and femur.