Rationale:

A model of medical device may include other medical devices as parts. Is is in most cases useful to answer qthe question "What are the devices parts of the current one?", rather than to answer the question "What device embedds the current one?" 

The data element parentDevice enables to answer the second question, not the first one. Moreover, a screw is a device that may be part of the composition of multiple devices. It is more important to know what are the screws used in the current device than the other way round. Hence the proposed addition below:

Proposed change:

Add an optional, repeatable devicePart BackboneElement defined as follows:

devicePart          0..*   BackboneElement            a device part of the current one

– deviceModel 1..1   Reference(DeviceDefinition)     the instance representing the part

– count             0..1  integer    the number of occurrences of the part in the current device