Introduction The innervation of the hip joint has gained attention in recent years, with research highlighting its relevance for treatments of hip osteoarthritis and its importance in contributing to pain and neuromechanics of the hip joint.¹ Hip osteoarthritis is a unique disorder, with a distinct etiopathology, high prevalence in the elderly, and it poses great burden on individual’s quality of life and society, as such early detection and management are paramount.²Total hip arthroplasty (THA) aims to resolve the associated pain, however, the success of this procedure is variable.³ One of the most common reasons for its failure is dislocation, therefore necessitating the requirement for revision surgery.³ Furthermore, this procedure is not suitable for all, particularly due to co-morbidities.⁴Radiofrequency ablation (RA) has emerged a suitable alternative in such cases, and involves targeting the nerves of the hip joint,⁴ thus understanding the detailed anatomy is important to inform these procedures. The aim of this work is to highlight the current knowledge on the hip joint innervation and surrounding structures, its clinical relevance, and future directions of research.

Detailed Anatomy of the Hip Joint Alongside the osseous stabilizers, the proximal femur and acetabulum, further passive supporting structures exist, namely the ligament of the head of the femur, the hip joint capsular complex, fascia and adipose tissues surrounding the hip.¹ The active stabilisers are the muscles of hip kinematics, while the neural components of the hip core complex of stability are the mechanoreceptive elements, free nerve endings and nerves, with various motor and sensory functions.¹ The original text of Hilton’s law states ‘The same trunks of nerves whose branches supply the groups of muscles moving a joint furnish also a distribution of nerves to the skin over the insertions of the same muscles; and—what at this moment more especially merits our attention—the interior of the joint receives its nerves from the same source.’⁵ While the literature highlights that this law appears to apply to the hip joint as the muscles surrounding the hip joint are innervated in a compartmental manner with specific branches to each muscle,⁶ and there is also evidence of sub-compartmental innervation within muscles, such as in the tensor fascia latae⁷ and gluteus medius.⁸ Additionally, the neural components of the hip capsule are also derived from nerves of the lumbosacral plexus,⁹ while the specific innervation of fascia, adipose tissue, the transverse acetabular ligament (TAL), ligament of the head of the femur (LHoF) and labrum remains unclear (table 1).^{1 10-13} The hip capsule,¹⁴ LhoF,¹⁰ labrum^{10 11} and TAL¹¹ were shown to contain free nerve endings and mechanoreceptors and therefore play a role in pain sensation and proprioception.

Table 1: Table demonstrating the origin of innervation for the tissues of the hip joint. LHoF=Ligament of the head of the femur. TAL=Transverse acetabular ligament.

The nuanced specifics of the distribution and prevalence of each nerve supplying the hip joint innervation is complex and variable.²⁰ Some nerves are well documented in relation to their course and related osteological landmarks or incisions sites, including the femoral,²¹ lateral femoral cutaneous,²² pudendal,²³ sciatic,²³ and superior gluteal nerves.²⁴ For example, the femoral nerve is located 19-27 mm from the anterior acetabular rim across its circumference,²¹ crucially understanding this facilitates application of this knowledge into clinical scenarios, such as when developing surgical approaches. Whereas the anatomy of smaller nerve branches is less well understood, these are known to be undulating, and course through hip capsule tissues in various directions.¹ This is thought to result in a discrepancy in the innervation density and distribution at microscopic and macroscopic levels.^{9 14} It remains unclear how nerves specifically course into each muscle, through adipose tissue, fascia, or into the bones of the hip joint.¹ Overall, these nerves are shown to innervate the tissues of the hip in different patterns between individuals.²⁰ Some differences in innervation patterns include a single or dual innervation of the pectineus with branches from the obturator and femoral nerves,¹⁶ differences in consensus on the distribution of nerves across the capsule,^{9 14} age and sex related changes in the course of the sciatic nerve,²⁵ which each can have potential functional implications if damaged. Despite these variations, generally the distal-medial aspect and the proximal-lateral aspect of the hip capsule appear regions of higher density, and likely receive innervation derived from the femoral and obturator nerves, but also potentially other sources.^{1 9} Despite this, greater focus has been on macroscopic nerve distribution,^{9 20} microscopic mechanoreceptor distribution¹ and therefore it remains unclear which regions contain greater density of free nerve endings alone.

Clinical Relevance and Applications The nerves supplying the hip joint or their neural elements may be targeted for regional anaesthesia,⁴ or avoided intraoperatively such as during THA, and arthroscopy,^{7 21} therefore understanding of their spatial distribution and variation is imperative to achieve desired outcomes. From a regional anaesthesia perspective, there are multiple methods to target the nerves of the hip joint, including a regional or specific nerve targeted approach.⁴ These are used in different scenarios, such as:

• Pre–operatively: deep posterior gluteal block for the posterior hip,²⁶ fascia iliaca block,²⁷ and femoral nerve block²⁷ for the anterior hip.

• Acute pain: PENG block to target the femoral nerve,²⁸ and iliopsoas plane block, which both provide a generally motor–sparing effect.²⁹

• Chronic pain: RA of the obturator, femoral nerve⁴ and branches to greater trochanter³⁰ are shown to be effective.

Whereas from a surgical perspective, understanding the nerve supply to the presents opportunities to identify areas to avoid nerves, such as the proximal-lateral hip capsule.¹ However, potential proprioceptive deficits may be minimal in all cases due to the significance of active and passive tissues in contributing to stability.¹ These deficits equally may be mitigated by repair of the hip capsule, which is shown to have favourable outcomes, including lower revision rates.³¹

Future Directions in Hip Joint Innervation Research To date, little is known about the innervation patterns of some associated joint tissues, and the specific distribution of free nerve endings within the hip capsule, which is important to develop recommendations for THA and RA. Given the anatomical variation of smaller capsular branches, that numerous nerves that supply the whole joint, and that the success rates of RA is variable, associated work should also determine appropriate sites for RA, lesion sizes, and necessity for multiple lesions to take into account the range of nerve variations surrounding the hip. Whereas, to enable developing anatomical informed THA approaches, future work should explore the physiological role of hip joint nerves to determine their relative contribution to joint stability and therefore the necessity for exploring this topic further to develop individualised pre-operative planning.

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