Femoral neck targeting
Femoral neck targeting is the process of calculating the centre of the femoral neck during hip resurfacing surgery. This can be done by hand or using electronic aides.
Early implant failure and mal-positioning of femoral components
Follow-up studies of metal-on-metal hip resurfacings have shown that some of the reasons for early implant failure are:- Mal-orientation of the implant stem in relation to the femoral neck axis.
- Notching of the femoral neck during femoral head preparation.
- Exposed cancellous bone after implantation.
If this is not achieved, and mal-positioning of the femoral component occurs, early implant failure can result.
Methods of femoral neck targeting
There are many methods surgeons use to calculate the true centre of the femoral neck. Today the methods broadly fall under two categories:- Navigational - which can be performed using computer tomography or fluoroscopy.
- By hand - using various alignment guides/ reference devices/ targeting devices to calculate the true centre of the femoral neck.
Below is a comparative analysis of both types of navigational targeting against traditional targeting techniques.
CALCULATIONS BY HAND | CT-based | FLUOROSCOPY |
+Less operative time needed | +Accurate in producing reproducible quality | +Little additional planning pre-operatively |
+Little additional learning curve | +3D feedback of anatomical landmarks | +Accurate in producing reproducible quality |
+Smaller, more manageable devices | -More operative time required | -Significant learning curve |
+Extensive long-term results available | -Pre-operative planning time-consuming | -More operative time required |
-Degrees of error more apparent due to required intra-operative calculations | -Ionizing radiation from X-rays pose potential health risk to patient |
A computer-assisted device to calculate the true centre of the femoral neck, without the above disadvantages, has not yet been created. The main issues with the current navigational systems are the levels of extra apparatus required in the operating theatre. In the case of fluoroscopy, a "C-arm" X-ray machine is used, which is a very large piece of apparatus. In the case of CT-based navigation, computer software and training in the use of such software is required, which again adds more apparatus to the operating room.
The popularity of CT and fluoroscopy-based navigation amongst surgeons has been increasing in recent years. The application of these techniques however has been proven to be best suited for different types of surgery. Fluoroscopy-based methods are easier to handle in routine cases with normal anatomy, or cases with lesser deformities. On the other hand, CT-scanning techniques have been better suited to cases of congenital and post-traumatic deformities. It is less suited to routine cases, due to the time-consuming pre-operative procedures e.g. setting up CT-scan, data transfer, planning.
The conventional "by hand" calculations, however are much more popular overall. Most hip resurfacing operative techniques come with some form of guide instrumentation to find the true centre of the femoral neck. These conventional kits will most likely be the instruments that the surgeon will use first, and perhaps become accustomed to. The two most important factors though are
- Conventional instrumentation is relatively more popular because it is small and hand-held, without the need for large pieces of apparatus.
- They are relatively cheaper to buy.