The present invention is directed to an improved system for controlled release of a bone growth promoting compound and to a flowable composition for its formation. The flowable composition is composed of a bone growth promoting compound, a thermoplastic polymer and an organic solvent. The flowable composition is capable of forming a biodegradable and/or bioerodible microporous, solid polymer matrix. The matrix is useful as an implant in patients (humans and animals) for delivery of a bone growth promoting compound to certain tissues.