Ossification of caroticoclinoid ligament and its clinical importance in skull-based surgery

ABSTRACT CONTEXT: The medial end of the posterior border of the sphenoid bone presents the anterior clinoid process (ACP), which is usually accessed for operations involving the clinoid space and the cavernous sinus. The ACP is often connected to the middle clinoid process (MCP) by a ligament known as the caroticoclinoid ligament (CCL), which may be ossified, forming the caroticoclinoid foramen (CCF). Variations in the ACP other than ossification are rare. The ossified CCL may have compressive effects on the internal carotid artery. Thus, anatomical and radiological knowledge of the ACP and the clinoid space is also important when operating on the internal carotid artery. Excision of the ACP may be required for many skull-based surgical procedures, and the presence of any anomalies such as ossified CCL may pose a problem for neurosurgeons. CASE REPORT: We observed the presence of ossified CCL in a skull bone. A detailed radiological study of the CCL and the CCF was conducted. Morphometric measurements were recorded and photographs were taken. The ACP was connected to the MCP and was converted into a CCF. Considering the fact that standard anatomy textbooks do not provide morphological descriptions and radiological evaluations of the CCL, the present study may be important for neurosurgeons operating in the region of the ACP.


INTRODUCTION
The medial end of the lesser wing of the sphenoid bone forms the anterior clinoid process (ACP). 1 The ACP provides attachment to the free margin of the tentorium cerebelli and is grooved medially by the internal carotid artery. 1 The ACP is joined to the middle clinoid process (MCP) by the caroticoclinoid ligament (CCL), which is sometimes ossifi ed.A dural fold extending between the anterior and middle clinoid processes or ossifi cation of the CCL may result in the formation of the caroticoclinoid foramen (CCF). 1 In neurosurgical operations, the ACP is usually accessed in order to gain entry into the clinoid space. 2 After the internal carotid artery leaves the cavernous sinus, it is related medially to the ACP.The presence of an ossifi ed CCL may form a potential site for compression of the internal carotid artery.Abnormal variations in the ACP may pose a risk while it is being removed in regional surgical procedures.Knowledge about the ossifi cation of the CCL may be immensely benefi cial for skull surgeons.Considering the fact that anatomy textbooks do not provide a detailed description of the anatomoradiological characteristics of the CCL or CCF, the present study may prove especially relevant to neurosurgeons and radiologists in day-to-day clinical practice.

CASE REPORT
The skull bones kept in the Department of Anatomy are prepared by means of initial washing, followed by autoclaving and treatment with ethylene oxide, and are fi nally freeze-dried.During routine osteology teaching for undergraduate medical students, we observed an anomalous CCL in a skull bone.Anomalous ossifi cation of the CCL was noted and the specimen was photographed (Figure 1).Appropriate morphometric measurements were recorded and a proper radiological evaluation was also conducted (Figure 2).The ossified CCL (shown by vertical arrows in Figure 1) was found to extend between the ACP (marked as "4" in Figure 1) and the MCP (marked with an vertical arrows in Figure 1), in the bone specimen.The ossifi ed CCL was found bilaterally, on both sides of the skull.The anterior clinoid processes on the two sides were separated by a distance of 2.1 cm.The anterior and posterior clinoid processes were separated by a distance of 0.7 cm and 0.6 m on the right and left sides, respectively.The sulcus chiasmaticus was situated at a distance of 0.5 cm behind the jugum sphenoidale.The right posterior clinoid process was found to be more prominent (marked as "6" in Figure 1) than on the left side.The ossifi ed CCL measured approximately 1.5 cm on each side.The CCF was prominently formed as a result of the presence of the ossifi ed CCL.The maximum transversal dimension between the optic canals on each side was 1.5 cm.

DISCUSSION
The ACP forms the attachment site for the free anterior margin of the tentorium cerebelli, whereas the MCP provides the attachment for the diaphragma sellae. 1 The parts of the sphenoid bone that are usually reported as capable of ossifi cation are the pterygospinous and interclinoid processes. 1s described in conventional textbooks of anatomy, the ACP may be joined to the MCP by a ligament or dural fold. 1 The bony bridge joining the ACP and MCP converts the distal end of the carotid sulcus into an ostium known as the CCL. 2 To the best of our knowledge, no single osteological study has ever been supplemented with additional radiological fi ndings of a CCL, and the present case is a humble attempt to highlight this fi nding.A skiagram was taken using separate wires inserted into two different openings, i.e. the optic canal and CCF, for easy differentiation (Figure 2).The internal carotid artery is present in the medial groove of the ACP and it may be compressed by the ossifi ed CCL, giving rise to vascular complications.The presence of an ossifi ed CCL is likely to cause compression and straightening of the internal carotid artery. 3In the present case, the clinical history of the patient was not available to corroborate this observation.
The internal carotid artery is conventionally divided into six segments and the clinoid segment of the artery is located between the proximal and distal dural rings. 4In any surgical operation involving exposure of the clinoid segment of the internal carotid artery, excision of the anterior clinoid process is mandatory.Even to expose the cavernous sinus superiorly and to manage paraclinoid aneurysm, the ACP has to be removed. 2,5The clustering of the neurovascular structures in the vicinity of the ACP renders the surgery more risky. 2Prior anatomical knowledge is essential for identifying any inadvertent injury to the internal carotid artery.
Research studies have also reported the fact that an ossifi ed CCL makes the removal of the ACP more diffi cult, especially in the presence of any aneurysm. 2Drilling of the ACP, when required, may cause inadvertent injury to the internal carotid artery and optic nerve.There have been previous reports of CCF occurrence in dried human skulls. 6That particular anatomical study laid less emphasis on the radiological aspects of the ossifi ed CCL. 6n comparison to earlier anatomical studies, the present study has not only described the morphological and clinical characteristics of an ossifi ed CCL, but also displayed the radiological features of an ossifi ed CCL, which may be benefi cial for radiologists.
Another important clinical characteristic is the pneumatization of the ACP, which has to be evaluated pre-operatively, in order to avoid serious complications like pneumocephalus and rhinorrhea. 7For any surgery involving the ACP, preoperative imaging may be advised, to keep such anomalies in view.Interestingly, 60% of ACP cases are pierced by narrow venous canals arising from the anterior cavernous sinus and traversing through the clinoid space.These are considered to be a potential source of bleeding during removal of the ACP. 8It must also be remembered that the extraocular nerves traverse to the superior orbital fi ssure inferolaterally to the ACP, and it is essential for surgeons to adopt a careful approach when operating on the ACP. 9 CONCLUSION If an ossifi ed CCL is present, it is likely to cause compression of the internal carotid artery.A detailed anatomical report of such an anomaly was presented in the present case.Radiological studies on the CCF and its differentiation from the optic canal may be clinically important for radiologists.Anatomical knowledge about ossifi cation of the CCL may be useful in cases of surgery involving removal of the ACP, for which additional risk is involved. 2

Figure 2 .
Figure 2. X ray photograph of skull (lateral view), with wires inside optic canal and caroticoclinoid foramen.a. Upper end of bent wire inside optic canal; b.Lower end of bent wire inside optic canal; c. Posterior end of bent wire inside caroticoclinoid foramen; d.Anterior end of bent wire inside caroticoclinoid foramen.The ossifi ed caroticoclinoid ligament is shown with an arrow.