Load sharing in parallel vs orthogonal screws

[maveric]

Hi I would be grateful for some help or pointers for the following question.
I am an orthopaedic surgeon and often when we fix fractures we use screws to hold the bone in place.

We use different configurations of screws (ie one or two parallel or orthogonal, two screws at right angles to each other) based on either the fracture pattern or sometimes we are driven to use this configuration based on the manufacturers implant designs such as in intramedullary nails, which are rods we pass into the bone to fix fractures. In intramedullary nails we pass the screws through the bone and nail in a manner that if effectively 'locks' the nail hence they are called locking screws. (http://orthopedics.about.com/cs/brokenbones/g/imrod.htm)

My question is if I am using two interlocking screws to fix a fracture how will the load be distributed between the screws if I use two parallel screws vs orthogonal screws ( screws at right angle to each other). What i am really after is can the load distribution on screws be defined based on its configuration in an engineering or mathematical sense.

I hope i make sense and any help would be appreciated.

Many thanks

 

[FredGarvin]

It's a bit tough to understand the geometry of what you deal with so perhaps we can start with some basics about fasteners...

A fastener for a joint has one major purpose. That is to create clamping force. The fasteners are usually NOT the members that carry the load in a joint. The frictional forces created by the clamping is responsible for the strength of the joint. Granted there are always exceptions and this is definitely not an absolute. However, the clamping force created in the joint is paramount to the joint's strength and the required sizing of the fasteners.

If there is any way you can provide a basic picture or diagram to discuss, that would be a huge help.