A more or less literal translation. Three native Catalans (including two engineers) were unable to understand what it means.

el ***pas de ròtula*** tangent a l’avinguda Diagonal
the hinge road lying at a tangent to Avinguda Diagonal

hinge joint in the human leg connecting the tibia and fibula with the femur and protected in front by the patella

http://www.thefreedictionary.com/patella

To hinge is defined as to attach or join to something using a mechanism that allows for swinging or opening, or to depend on entirely or to turn on entirely.
When an ironing board is attached to a wall by a mechanism that allows it to fold up, this is an example of a time when the ironing board hinges to the door.
When a project depends upon receiving funding, this is an example of a time when the project hinges on the receipt of funding.

http://www.yourdictionary.com/hinge



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Note added at 1 hr (2014-05-25 16:34:19 GMT)
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This dale is a milestone in the heights that acts as a hinge between the plains of the puna and those of the Calchaquí Valley.

http://www.welcomeargentina.com/salta/excursion-the-acay-dal...

This large tree-lined hard-paved plaza is the formal area within Putrajaya and acts as a spatial hinge between the Boulevard axis and the Government Precinct axis.

http://www.kiat.net/putra/core1.html



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Note added at 1 hr (2014-05-25 16:39:55 GMT)
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Or you could put 'Thus, Sant Pere Màrtir acts as a hinge that lies at a tangent to Avinguda Diagonal'.