Femoral Triangle Versus Adductor Canal
The Most Reliable Location to Block Nerve to Vastus Medialis.
A Cadaveric Study.
Johnston DF§, Black N*, Cowden R‡, Turbitt L§, Taylor S†
Compared to femoral nerve block, an adductor canal (AC) block has the desirable effect of preserving quadriceps function while maintaining a similar analgesic profile after total knee arthroplasty (TKA).1
The nerve to vastus medialis (NVM) supplies sensation to important structures relevant for TKA (e.g. the medial knee capsule, medial retinaculum, ligamentous intraarticular structures and the undersurface of the patella).2
There is variation in the literature findings whether the NVM is reliably located within the AC.3,4 This is further complicated by mid-thigh injections being termed AC blocks when they are in fact injections at the level of the femoral triangle (FT).5
The objective of this cadaveric study is to determine the effect of location of ultrasound guided injections on achieving successful spread to the NVM when comparing AC to FT sites.
MATERIALS + METHODS
Four unembalmed fresh-frozen cadavers were used for this study carried out at the Biomedical Sciences department, Queen's University, Belfast in June 2017.
Consent had previously been obtained from each of the donors whose remains were used for this study.
Each cadaver acted as its own control with one leg undergoing FT injection and the other AC injection according to US landmarks.
FT injection was placed just proximal to the point at which medial borders of sartorius and adductor longus muscles appose (Fig.1). AC injection was placed 1cm proximal to the descent of the femoral artery through the adductor hiatus (Fig.2).
Twenty millilitres of a mixture of ink and methylcellulose stabiliser was deposited at the '11 o'clock' position of the femoral artery using an identical technique. Standardised approach to dissection took place one hour later to observe extent of staining to the NVM.
In all specimens where the injectate was introduced into the FT, both saphenous nerve (SN) and NVM were stained (Fig. 3). In all specimens where the AC injection was performed, the NVM was spared and only the SN was stained (Fig. 4).
Using reproducible sonoantomical landmarks, dye injected into the AC did not spread to the NVM, whereas the NVM was consistently stained with FT injections.
The proximal margin of the vastoadductor membrane was consistently located between the injection points which could explain why proximal spread was inhibited.
Clinical studies comparing FT versus AC blocks may demonstrate reduced pain after TKA with the more proximal injection of local anaesthetic facilitating better spread to the NVM.
1. Gao F et al. Clin J Pain. 2017;33(4):356-368.
2. Guild GN et al. Orthop Clin N Am 2015;46:1-8
3. Burckett-St Laurant D et al. Reg Anesth Pain Med 2016; 41:321-7.
4. Horner GH et al. Clin Orthop Relat R 1994;301:221-226.
5. Bendtsen TF et al. Reg Anesth Pain Med. 2014;39:253–254.
§ Consultant Anaesthetist, Belfast Health & Social Care Trust, Belfast, UK
* Specialist Trainee in Anaesthesia, Craigavon Area Hospital, Southern Trust
‡ Medical Student, Queen’s University Belfast, UK
† Queen's University Belfast, Centre for Biomedical Sciences Education, Belfast, UK