A bone anchor assembly (100, 100') for stabilizing bones or bone fragments, the bone anchor assembly (100, 100') comprising: a bone anchor including a head portion, wherein the head portion includes a first tool interface for engaging a first surgical instrument operatively associated with the bone anchor; a body portion including a longitudinal axis, an upper end with an upper opening, a lower end with a lower opening, a bore extending between the upper opening and lower opening along the longitudinal axis, and an integrated longitudinal spinal rod extending from the body portion substantially perpendicular to the longitudinal axis, the bore including a lower edge portion and an enlarged diameter portion proximate to the lower edge portion and between the lower edge portion and the upper end; and a collet movably positioned within the bore of the body portion, the collet including a first end and a second end, wherein the head portion of the bone anchor is enlarged and curvate to facilitate pivoting or polyaxial movement of the head portion with respect to the collet, characterized in that the collet further includes at least two slots extending from the second end, the at least two slots defining flexible arms, the flexible arms rendering the collet expandable to accept the head of the bone anchor and compressible to secure the head of the bone anchor relative to the collet, the collet further including a cavity, wherein the flexible arms of the collet are positioned in alignment with the enlarged diameter portion in a loading position so that the head of the bone anchor can be received in the cavity formed in the collet, and wherein at least a portion of the flexible arms contact the lower edge portion in a locked position so that the head of the bone anchor is secured with respect to the collet, and in that the lower edge portion has a first spherical surface proximate the lower end, and the enlarged diameter portion has a second spherical surface. a bone anchor including a head portion, wherein the head portion includes a first tool interface for engaging a first surgical instrument operatively associated with the bone anchor; a body portion including a longitudinal axis, an upper end with an upper opening, a lower end with a lower opening, a bore extending between the upper opening and lower opening along the longitudinal axis, and an integrated longitudinal spinal rod extending from the body portion substantially perpendicular to the longitudinal axis, the bore including a lower edge portion and an enlarged diameter portion proximate to the lower edge portion and between the lower edge portion and the upper end; and a collet movably positioned within the bore of the body portion, the collet including a first end and a second end, wherein the head portion of the bone anchor is enlarged and curvate to facilitate pivoting or polyaxial movement of the head portion with respect to the collet, characterized in that the collet further includes at least two slots extending from the second end, the at least two slots defining flexible arms, the flexible arms rendering the collet expandable to accept the head of the bone anchor and compressible to secure the head of the bone anchor relative to the collet, the collet further including a cavity, wherein the flexible arms of the collet are positioned in alignment with the enlarged diameter portion in a loading position so that the head of the bone anchor can be received in the cavity formed in the collet, and wherein at least a portion of the flexible arms contact the lower edge portion in a locked position so that the head of the bone anchor is secured with respect to the collet, and in that the lower edge portion has a first spherical surface proximate the lower end, and the enlarged diameter portion has a second spherical surface.