Volkmann canal

**Definition and structure** - Volkmann canals (perforating canals) are transverse or oblique channels within cortical (compact) bone. - They interconnect neighboring Haversian canals, which run longitudinally, parallel to the long axis of the bone. They also connect Haversian systems (osteons) to the periosteum and the medullary cavity. - They contain neurovascular bundles: small arteries and veins, lymphatics, and nerve fibers. - They are not lined by concentric lamellae. This is the classic histologic distinction from Haversian canals, which are surrounded by concentric lamellae to form an osteon.

**Haversian vs. Volkmann canals (high-yield comparison)** - Haversian canal: runs longitudinally (parallel to the bone axis) and sits at the center of an osteon. It is surrounded by concentric lamellae. - Volkmann canal: runs transversely or obliquely (perpendicular to the Haversian canals). It has no concentric lamellae around it, and it connects Haversian canals to each other and to the periosteum and marrow.

**Physiologic role** - They provide the vascular and nutrient link between the periosteal and endosteal blood supplies and the interior of cortical bone. - Osteocytes in lacunae receive nutrients through canaliculi that radiate from the Haversian canals. The Volkmann canals deliver the vessels that feed those Haversian systems. - Blood supply to long bones comes from the nutrient artery, periosteal arteries, and metaphyseal/epiphyseal arteries. The Volkmann canals are the conduits through which periosteal vessels reach the cortex.

**Clinical significance** - They are a route for vascular spread in bone. In hematogenous osteomyelitis, infection can extend from the medullary cavity through Volkmann and Haversian canals to the cortex, then to the subperiosteal space, forming a subperiosteal abscess. Elevation of the periosteum compromises the periosteal blood supply and contributes to bone necrosis (sequestrum), with new bone formation around it (involucrum). - Rarely, they allow tumor or infection to spread through the cortex. - Eponym: named after Alfred Wilhelm Volkmann, a German surgeon. Do not confuse it with Volkmann ischemic contracture, which is a different entity. That is a forearm compartment syndrome complication, classically after supracondylar humerus fracture, that leads to fibrosis and flexion contracture of the wrist and fingers.

**Board pearls** - Think "Volkmann = Vertical (perpendicular) to the Vessels' long axis". Haversian = parallel (Having long axis). - Osteon (Haversian system) = Haversian canal + concentric lamellae + osteocytes in lacunae + canaliculi. - Interstitial lamellae are remnants of old osteons. Circumferential lamellae lie at the outer (periosteal) and inner (endosteal) surfaces. - Compact bone is organized into osteons, while trabecular (cancellous) bone is organized into trabeculae without Haversian systems.