What is Dr. Zubair Khalid's research focus?

Dr. Zubair Khalid specializes in molecular virology, mRNA vaccine development, and computational biology, with a focus on avian pathogens like IBDV and Avian Reovirus.

Where is Dr. Zubair Khalid currently working?

Dr. Zubair Khalid is a Postdoctoral Research Associate at the University of Maryland (UMD), specifically within the Department of Animal and Avian Sciences.

Pelvic Anatomy: Comparative Guide for Veterinary Study

Name: Pelvic Anatomy: Comparative Guide for Veterinary Study
Creator: Dr. Zubair Khalid

By Dr. Zubair Khalid, DVM, MS, PhD · 2026-09-20

The bony pelvis is a ring formed by three paired bones (ilium, ischium, and pubis) that fuse at the acetabulum, joined dorsally to the sacrum and the first caudal vertebrae. Across domestic species the same bones appear, but their orientation, the shape of the pelvic inlet, and the depth of the ischiatic arch differ enough that a dog, a horse, and a cow each have a recognizable pelvic silhouette.

This article walks through the osteology, the joints, the sex differences, and the species-specific landmarks that matter in clinical practice. Pelvic anatomy is the foundation for radiography, obstetrics, nerve blocks, and orthopedic surgery, so the comparative differences are worth learning precisely.

This article is educational and is not a substitute for veterinary diagnosis or treatment.

The Bony Pelvis: Core Osteology

The Three Paired Bones

The ilium is the largest and most cranial of the three bones. Its flat iliac wing faces craniodorsally and its narrow body (or shaft) runs caudoventrally to meet the other two bones at the acetabulum. The wing carries a rough auricular surface that articulates with the sacrum at the sacroiliac joint, and a tuber coxae (the cranial dorsal iliac spine in the dog) that is the palpable point of the hip.

The ischium forms the caudal part of the pelvic floor and the caudal wall of the pelvis. Its caudal border is the ischiatic arch, the concave notch between the two ischiatic tuberosities. The ischiatic arch is the landmark that radiologists use to judge pelvic canal depth and to detect fractures of the caudal pelvis.

The pubis is the smallest and most ventral bone. It contributes the cranial part of the pelvic floor, forms the pubic symphysis with its opposite number, and gives origin to the prepubic tendon. The pecten (pecten pubis) is the sharp ridge on the dorsal surface of the pubis that serves as a surgical and anatomical landmark.

All three bones meet at the acetabulum, the cup-shaped socket of the hip joint. The obturator foramen is the large oval hole enclosed by the ischium and pubis, transmitting the obturator nerve and vessels.

The Sacrum and First Caudal Vertebrae

The sacrum is formed by fusion of several sacral vertebrae (typically three in the dog, five in the horse, and five to six in ruminants). Its wings articulate with the ilium at the sacroiliac joints, and its ventral surface contributes to the dorsal wall of the pelvic canal. The first caudal vertebra continues the dorsal boundary caudally. In dogs, lumbosacral transitional vertebrae occur at the junction of the last lumbar vertebra and the sacrum, and their prevalence varies sharply by breed. In a study of 1,679 dogs screened through a national hip dysplasia program, lumbosacral transitional vertebra prevalence was 47.8% in Brittany dogs and 14.9% in Danish-Swedish Farmdogs [1]. That figure matters clinically because lumbosacral transitional vertebrae are associated with caudal spine changes and can complicate radiographic interpretation of the pelvis.

Joints of the Pelvis

The pelvis has three clinically significant joints:

Sacroiliac joint. A tight synovial joint between the auricular surface of the ilium and the wing of the sacrum. Motion is minimal, and the strong dorsal and ventral sacroiliac ligaments resist shear.
Pubic symphysis. A fibrocartilaginous joint between the two pubic bones. In ruminants the symphysis is more completely fused than in the dog, which is why the ruminant pelvis behaves more like a rigid ring.
Hip joint (coxofemoral joint). A ball-and-socket joint between the femoral head and the acetabulum. The round ligament (ligament of the head of the femur) and the transverse acetabular ligament stabilize it, and the acetabular labrum deepens the socket.

The prepubic tendon is a composite structure at the cranial border of the pubis. In the horse and ruminants it is built from crossed and uncrossed tendons of the pectineus muscles, the pelvic tendons of the rectus and obliquus abdominis muscles, and tendons of the cranial gracilis muscles [2]. In the dog the arrangement differs in four respects: the pectineus tendons do not cross, an iliopubic cartilage is intercalated on each side, the transversus abdominis aponeurosis may join the tendon in some dogs, and the gracilis tendon does not extend to the prepubic tendon [2]. That anatomical difference explains why prepubic tendon rupture presents differently in horses and cows than in dogs.

Comparative Pelvic Anatomy Across Domestic Species

Dog and Cat

Dogs and cats share a narrow pelvic canal relative to body size. The pelvic inlet is roughly oval and the ischiatic arch is prominent and deep, giving the caudal pelvis a distinct notch on ventrodorsal radiographs. The iliac wings are steeply angled and the sacrum is short.

The dog pelvis is the most studied in veterinary practice. Radiographic pelvic measurements are used in hip dysplasia screening, and femoral measurements from pelvic radiographs can classify sex and species. In a machine-learning study of 280 animals (140 dogs and 140 cats), the most influential variables for sex classification were femoral length in cats and left intercondylar fossa width in dogs, while inter-femoral-head distance was the most influential variable for species classification [3]. That study also validated a technique for creating simulated radiographs from CT datasets, and no significant differences were found between simulated and actual radiographs for canine pelvis and feline abdomen when board-certified radiologists scored anatomical landmark visualization and patient positioning [4]. For students, that means simulated pelvic radiographs are a legitimate study tool.

The cat pelvis is smaller and more slender than the dog's, with a narrower canal and a more pronounced ischiatic arch. Pelvic landmarks guide regional anesthesia in cats. In a cadaveric pilot study of pudendal nerve block, a blind approach based on palpable bony landmarks lateral to the sacrococcygeal joint achieved an 87.5% success rate (7/8) versus 75% (6/8) for the ultrasound-guided technique [5]. The ischial nerve was inadvertently stained in 75% (6/8) of blind injections and 25% (2/8) of ultrasound-guided injections, which is a reminder that the ischium and sacrococcygeal region are crowded with clinically important structures [5].

Horse

The horse has the most distinctive pelvis among domestic species. The ilium is long and steeply inclined, and the pelvic inlet is wide compared with the dog. The tuber coxae is prominent and palpable, and the tuber sacrale sits close to the sacral midline. The wide inlet and the relatively roomy canal reflect the horse's size and the mechanics of foaling.

The prepubic tendon in the horse incorporates crossed and uncrossed pectineus tendons plus the pelvic tendons of the rectus and obliquus abdominis muscles, and the pelvic attachments of the linea alba and the yellow abdominal tunic are incorporated into it [2]. This composite structure is not a transverse ligament and is not homologous to the human superior pubic ligament [2]. Prepubic tendon rupture in mares is a recognized periparturient emergency, and the anatomy explains why the rupture site is cranial to the pubis.

Ruminants

Cattle, sheep, and goats have a more vertical ilium than the dog and a fused pubic symphysis. The pelvis is longer and narrower than the horse's, and the pelvic inlet is more oval. The vertical ilium means the tuber coxae sits higher and more cranially relative to the tuber ischiadicum, which changes the angle of the pelvic canal and affects calving mechanics.

The ruminant prepubic tendon follows the horse pattern: crossed and uncrossed pectineus tendons, pelvic tendons of the rectus and obliquus abdominis, and cranial gracilis tendons, with incorporation of the linea alba and yellow abdominal tunic [2]. The fused symphysis makes the ruminant pelvis a rigid ring, which is why fractures of the pelvis in cattle tend to be complete rather than partial.

Species Comparison Table

Species	Ilium orientation	Pelvic inlet shape	Key landmark
Dog	Steeply angled, moderate length	Oval, narrow	Prominent, deep ischiatic arch
Cat	Steeply angled, slender	Oval, narrow	Sharp ischiatic arch, sacrococcygeal joint
Horse	Long, steeply inclined	Wide, roomy	Prominent tuber coxae, wide canal
Ruminant	More vertical	Oval, long and narrow	Fused pubic symphysis, vertical ilium

Sex Differences in the Pelvis

The female pelvis is wider than the male pelvis in all domestic species, with a larger pelvic inlet and a more obtuse pubic angle. These differences support parturition and are visible on radiographs and on gross dissection.

In the female, the iliac wings flare more laterally, the pelvic inlet is broader, and the ischiatic arch is shallower. The pubic angle (the angle between the two pubic bones at the symphysis) is more obtuse in females, which widens the pelvic floor and gives the fetus more room to pass. In the male, the pelvis is narrower, the inlet is smaller, and the pubic angle is more acute.

Sex differences can be quantified radiographically. In the machine-learning study of dogs and cats, the extra trees classifier achieved the highest accuracy for sex classification (0.79 in dogs and 0.75 in cats) using nine radiographic femoral measurements [3]. That level of accuracy is useful for forensic and zooarchaeological work but is not a substitute for clinical judgment in a live animal.

The sex differences matter clinically in three situations. First, dystocia risk is higher in females with a narrow inlet or a previous pelvic fracture that narrowed the canal. Second, pelvic radiography for hip dysplasia must account for sex-related shape differences when comparing against breed standards. Third, surgical planning for cesarean section or pelvic fracture repair depends on knowing whether the patient is male or female.

Clinical Relevance, Limitations and Common Mistakes

Pelvic anatomy shows up in practice in several ways.

Radiographic interpretation. The ventrodorsal pelvic view is standard for hip dysplasia screening, and the ischiatic arch, obturator foramina, and acetabula are the landmarks radiologists check first. Lumbosacral transitional vertebrae can mimic fractures or obscure the sacroiliac joint, and their prevalence varies by breed [1]. Simulated radiographs generated from CT datasets can help students learn these landmarks without exposing animals to repeated radiation [4].

Regional anesthesia. The pudendal nerve block in cats relies on palpable bony landmarks around the sacrococcygeal joint and ischium [5]. The ischial nerve is close by, and inadvertent staining occurs in a substantial fraction of injections, so ultrasound guidance or careful technique is warranted [5].

Surgery. The prepubic tendon is a key structure in hernia repair and in cesarean section closure. Its species-specific composition means that closure technique cannot be copied directly from one species to another [2]. In dogs, the iliopubic cartilage and the variable contribution of the transversus abdominis aponeurosis change the tissue available for repair [2].

Orthopedics. The ventromedial hip region contains the pectineus, iliopsoas, adductor longus, adductor magnus et brevis, gracilis, and external obturator muscles, all of which contribute to coxofemoral stability [6]. High-frequency ultrasound and MRI can identify all six muscles consistently, and ultrasound-guided dye injection followed by dissection confirmed accurate identification in a cadaveric study [6]. That protocol is useful for teaching and for planning soft tissue approaches to the hip.

Obstetrics. The pelvic inlet dimensions and the pubic angle determine whether a fetus can pass. In ruminants the fused symphysis and vertical ilium make the canal rigid, so a fetus that is too large or malpositioned is more likely to require surgical intervention.

Common Mistakes

Assuming the pelvis is the same shape across species. The dog's narrow canal and prominent ischiatic arch are not the horse's wide inlet and long ilium.
Ignoring sex when interpreting pelvic radiographs. The female inlet is larger and the pubic angle more obtuse, and these differences are visible on standard views.
Treating the prepubic tendon as a single structure. Its composition differs between the dog and the horse or ruminant, and repair technique must match the species [2].
Confusing the ischiatic arch with the pelvic inlet. The arch is the caudal notch, and the inlet is the cranial opening. They are different landmarks with different clinical implications.
Overlooking lumbosacral transitional vertebrae. These are common in some breeds and can be mistaken for fractures or for sacroiliac disease [1].

What Is Still Uncertain

The relationship between pelvic shape and locomotor performance is well described in some taxa but less so in domestic species. Comparative studies in lizards and anurans show that pelvic girdle shape tracks ecology and locomotion [7][8], and similar principles likely apply to dogs and cats, but the specific quantitative relationships in domestic mammals are not fully mapped. The functional consequences of sex-related pelvic shape differences for athletic performance in dogs and horses are also not well established.

Frequently Asked Questions

What bones make up the pelvis in dogs and cats?

The pelvis is made of the ilium, ischium, and pubis on each side, plus the sacrum and the first caudal vertebrae dorsally.

How does the horse pelvis differ from the dog pelvis?

The horse has a long, steeply inclined ilium and a wide pelvic inlet, while the dog has a narrow canal and a prominent ischiatic arch.

Why is the female pelvis wider than the male pelvis?

The female pelvis is wider to support parturition, with a larger pelvic inlet and a more obtuse pubic angle.

What is the ischiatic arch?

The ischiatic arch is the concave notch between the two ischiatic tuberosities at the caudal end of the pelvis.

Do ruminants have a fused pubic symphysis?

Yes, ruminants have a more completely fused pubic symphysis than dogs, making the pelvis a rigid ring.

What is the prepubic tendon?

The prepubic tendon is a composite structure at the cranial border of the pubis formed from pectineus, rectus abdominis, obliquus abdominis, and gracilis tendons, and its composition differs between dogs and horses or ruminants [2].

Can pelvic shape be used to determine sex in dogs and cats?

Radiographic femoral measurements can classify sex with moderate accuracy, about 0.79 in dogs and 0.75 in cats in one machine-learning study [3].

Why do lumbosacral transitional vertebrae matter in pelvic radiography?

They are common in some breeds and can be mistaken for fractures or obscure the sacroiliac joint, so they must be recognized during interpretation [1].