STOCKHOLDERS' AGREEMENT

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Exhibit 4.2

STOCKHOLDERS' AGREEMENT

ALPHATEC HOLDINGS, INC.
formerly known as
AMI ACQUISITION I CORP.

         THIS STOCKHOLDERS' AGREEMENT , originally dated as of March 17, 2005 (this " Agreement "), is by and among Alphatec Holdings, Inc. (formerly know as AMI Acquisition I Corp.), a Delaware corporation (the " Company "), HealthpointCapital Partners, LP (the " Fund "), and the investors as may from time to time be listed on Schedule I attached hereto (the Fund and such investors are collectively referred to herein as the " Investors ").

RECITALS

         WHEREAS, the Investors are beneficial owners of the capital stock of the Company; and

         WHEREAS, the Investors desire to enter into this Agreement to regulate their relationships with regard to the Company and to regulate their relationships among themselves as shareholders of the Company.

         NOW, THEREFORE , in consideration of the premises and the mutual agreements contained herein, the parties hereto agree as follows:

         SECTION 1.     Definitions.     

        1.01.     Certain Definitions.     

        (a)   As used herein, the following capitalized terms shall have the following meanings:

        " Affiliate " means any corporation, firm, limited liability company, partnership or other entity which directly or indirectly controls or is controlled by or is under common control with a party. For purposes of this definition, "control" means ownership, directly or through one or more Affiliates, of more than fifty percent (50%) of the shares of stock entitled to vote for the election of directors, in the case of a corporation, or more than fifty percent (50%) of the equity interests in the case of any other type of legal entity, status as a general partner in any partnership, or any other arrangement whereby a party controls or has the right to control the board of directors or equivalent governing body of a corporation or other entity.

        " Board " means the board of directors of the Company.

        " Bring-Along Sale " has the meaning ascribed to such term in Section 4.03.

        " Common Stock " means, collectively, the Series A Common Stock,, the Series A-1 Common Stock, the Series B Common Stock and the Rolling Common Stock, each par value $0.0001 per share, of the Company.

        " Company Indemnified Party " has the meaning ascribed to such term in Section 5.05(b).

        " Cutback Registration " means any Piggyback Registration to be effected as a Public Offering in which the managing underwriter with respect thereto advises the Company and the Requesting Holders in writing that, in its opinion, the number of shares of capital stock requested to be included in such registration (including shares of capital stock of the Company which are not Registrable Securities) exceed the number which can be sold in such offering without a material reduction in the selling price anticipated to be received for the shares of capital stock to be sold in such Public Offering.

        " Exchange Act " means the Securities Exchange Act of 1934, as amended, or any similar Federal statute, and the rules and regulations of the Commission thereunder, all as the same shall be in effect at any time or from time to time.

        " Financing " means any loan from a Person to the Company pursuant to which the Company issues a promissory note or similar instrument, or shares of capital stock (or warrants, options or other securities convertible into, or exchangeable for, shares of capital stock), or a combination thereof, to such Person.

        " Fund Shares " means any and all shares of Common Stock and Preferred Stock held by the Fund or its Permitted Transferees.

        " Holder " means any owner (beneficial or of record) of any Restricted Securities.

        " Holder Indemnified Party " has the meaning ascribed to such term in Section 5.05(a).

        " Indemnified Party " means any Company Indemnified Party or Holder Indemnified Party, as the case may be, that is entitled to indemnity pursuant to Section 5.05.

        " Indemnifying Party " means a party that is obligated to provide indemnification pursuant to Section 5.05.

        " Joinder Agreement " has the meaning ascribed to such term in Section 4.01(c).

        " Losses " has the meaning ascribed to such term in Section 5.05(a).

        " Permitted Transferee " has the meaning ascribed to such term in Section 4.01(c).

        " Person " means any individual, corporation, company, voluntary association, partnership, joint venture, trust or unincorporated organization, business, entity or government (or any agency, instrumentality or political subdivision thereof).

        " Piggyback Registration " means any registration of shares of capital stock of the Company under the Securities Act, whether for sale for the account of the Company or for the account of any holder of shares of capital stock of the Company (other than Registrable Securities).

        " Plan " means any stock option, stock grant, equity incentive or similar plan adopted by Board, pursuant to which the Company issues shares of, or options to purchase, capital stock of the Company to its employees, directors or consultants.

        " Preferred Stock " means, collectively, the Series A Preferred Stock, the Series A-1 Preferred Stock and the Series B Preferred Stock, each par value $0.001 per share, of the Company.

        " Proportionate Share " has the meaning ascribed to such term in Section 4.02(b).

        " Proposed Purchaser " has the meaning ascribed to such term in Section 4.02(b).

        " Public Offering " means an underwritten public offering of shares of Common Stock.

        " Registrable Securities " means any shares of Common Stock. As to any particular Registrable Securities, once issued such securities shall cease to be Registrable Securities when (i) a registration statement with respect to sale of such securities shall have become effective under the Securities Act and such securities shall have been disposed of in accordance with such registration statement, (ii) they shall have been distributed to the public pursuant to Rule 144, or (iii) they shall have ceased to be outstanding.

        " Registration Expenses " means all expenses incident to the Company's performance of or compliance with its obligations under this Agreement to effect the registration of Registrable Securities in a Piggyback Registration, including, without limitation, all registration, filing, securities exchange listing, fees of the National Association of Securities Dealers, all registration, filing, qualification and other fees and expenses of complying with securities or bleu sky laws, all word processing, duplicating and printing expenses, messenger and delivery expenses, the fees and disbursements of counsel for the Company and of its independent public accountants, including the

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expenses of any special audits or "cold comfort" letters required by or incident to such performance and compliance, the reasonable fees and disbursements of a single counsel and single firm of accountants retained by any Holder, premiums and other costs of policies or insurance against liabilities arising out of the Public Offering of the Registrable Securities being registered and any fees and disbursements of underwriters customarily paid by issuers or sellers of securities, but excluding underwriting discounts and commissions or transfer taxes, if any, in respect of Registrable Securities, which shall be payable by any Holder; provided, however, that, in any case where Registration Expenses are not to be borne by the Company, such expenses shall not include salaries of the Company's personnel or general overhead expenses of the Company, auditing fees, premiums or other expenses relating to liability insurance required by underwriters of the Company or other expenses for the preparation of financial statements or other data normally prepared by the Company in the ordinary course of its business or which the Company would have incurred in any event.

        " Restricted Securities " means any shares of Common Stock or Preferred Stock.

        " Rule 144 " means Rule 144 promulgated by the Commission under the Securities Act (or any successor or similar rule then in force).

        " Shares " means shares of Common Stock and Preferred Stock.

        " Subsidiary " means any corporation of which at least a majority of the outstanding shares of stock having by the terms thereof ordinary voting power to elect a majority of the board of directors of such corporation (irrespective of whether or not at the time stock of any other class or classes of such corporation shall have or might have voting power by reason of the happening of any contingency) is at the time directly or indirectly owned or controlled by the Company or by one or more of the Subsidiaries of the Company.

        " Tag-Along Purchase Offer " has the meaning ascribed to such term in Section 4.02(b).

        " Tag-Along Sale " has the meaning ascribed to such term in Section 4.02(b).

        (b)   References herein to the Common Stock outstanding "on a fully diluted basis" at any time means, without duplication, the number of shares of Common Stock then issued and outstanding, assuming full conversion, exercise and exchange of all securities of any type that shall be (or may become) exchangeable for, or exercisable or convertible into, Common Stock.

        (c)   References to "include" and "including" shall be construed as if followed by the phrase ", without being limited to,".

        1.02.     General Principles of Construction.     Unless otherwise specified herein, (a) "Section" shall refer to a section or subsection of this Agreement, as the context shall require, (b) "herein", "hereunder", "hereby", "hereof" and words of similar import shall refer to this Agreement and any schedules and exhibits hereto taken as a whole, and not just to the specific section in which such term appears, and (c) terms defined in the singular have the appropriate correlative meaning when used in the plural and vice versa.

         SECTION 2.     Corporate Governance.     

        2.01.     Number and Identity of Directors.     

        (a)   The number of directors of the Company shall initially be one (1). Such number may be increased or decreased, at any time, by the Board.

        (b)   Each Holder agrees that all directors of the Company shall be person nominated, from time to time, by the Holders of a majority of outstanding Fund Shares.

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        2.02.     Further Assurances.     Each of the Holders agrees to vote, in person or by proxy, all of the Shares owned by such Holders, at any annual or special meeting of the stockholders of the Company called for the purpose of voting on the election of directors, or by consensual action of Holders without a meeting with respect to the election of directors, in favor of the election of the directors nominated in accordance with this Section. Each Holder shall vote the Shares owned by such Holder and shall take all other actions necessary to ensure that the Certificate of Incorporation and By-laws of the Company do not at any time conflict with the provisions of this Agreement.

         SECTION 3.     Certificate of Incorporation.     

        3.01.     Amended Certificate.     The parties hereto acknowledge and agree that in connection with the execution of this Agreement, the Certificate of Incorporation shall be amended to read in its entirety as set forth in Exhibit A hereto.

         SECTION 4.     Transfers of Restricted Securities.     

        4.01.     Restrictions on Transfers; Permitted Transferees     

        (a)   Each Holder, severally and not jointly, agrees and acknowledges that such Holder will not, directly or indirectly, offer, sell, assign, pledge, encumber or otherwise transfer any Restricted Securities or solicit any offers to purchase or otherwise acquire or make a pledge of any Restricted Securities, except in the case of (i) a sale of Restricted Securities pursuant to an effective registration statement under the Securities Act, (ii) a Tag-Along Sale or (iii) a Bring-Along Sale, in each case otherwise in accordance with this Agreement.

        (b)   Except as specifically contemplated hereby, no Holder shall grant any proxy or enter into or agree to be bound by any voting trust with respect to any Restricted Securities nor shall any Holder enter into any stockholder agreements or arrangements of any kind with any Person with respect to any Restricted Securities inconsistent with the provisions of this Agreement (whether or not such agreements and arrangements are with other Holders of Restricted Securities who are not parties to this Agreement), including but not limited to, agreements or arrangement with respect to the acquisition, disposition or voting of Restricted Securities, nor shall any Holder act, for any reason, as a member of a group or in concert with any other Persons in connection with the acquisition, disposition or voting of Restricted Securities in any manner which is inconsistent with the provisions of this Agreement.

        (c)   None of the restrictions contained in this Agreement with respect to transfers of Restricted Securities shall apply with respect to any transfer or assignment by a Holder to an Affiliate of such Holder, provided such Holder obtains the prior written of the Company to such transfer or assignment (which consent shall not be unreasonably withheld), and provided further that such transferee or assignee (a " Permitted Transferee ") shall have executed and delivered to the Company, as a condition precedent to any acquisition of Restricted Securities, a Joinder Agreement substantially in the form of Exhibit B hereto (a " Joinder Agreement ") confirming that such Permitted Transferee takes such Restricted Securities subject to all the terms and conditions of this Agreement and agrees to be bound by the terms thereof. The Company shall not transfer upon its books any Restricted Securities to any Person except in accordance with this Agreement. For purposes hereof, the Permitted Transferees of a Holder shall include the Permitted Transferees of such Holder's Permitted Transferees.

        4.02.     Tag-Along Right.     

        (a)   If any Holder, or group of Holders, of Fund Shares proposes in a single transaction or a series of related transactions to sell, dispose of or otherwise transfer (except a transfer pursuant to Section 4.01(c) or Rule 144) any Restricted Securities then outstanding (on a fully diluted basis), such Holder or group of Holders shall refrain from effecting such transaction unless, prior to the

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consummation thereof, the other Holders shall have been afforded the opportunity to join in such transfer as provided in Section 4.02(b).

        (b)   Prior to the consummation of any transaction subject to the terms of this Section 4.02, the Person or group of Persons (the " Proposed Purchaser ") that proposes to acquire Restricted Securities in a transaction subject to Section 4.02(a) (the " Tag-Along Sale ") shall offer (the " Tag-Along Purchase Offer ") in writing to the other Holders the right to sell out of the total number of Restricted Securities proposed to be acquired in the Tag-Along Sale the same proportion (a " Proportionate Share ") of the number of Restricted Securities to be sold as the total number of Restricted Securities owned by the other Holders bears to the total number of Restricted Securities held on such date by the other Holders and the selling Holder or group of Holders, at the some price and on the same terms and conditions as the Proposed Purchaser has offered to such original selling Holder or group of Holders. Each other Holder shall have fifteen (15) days from the receipt of the Tag-Along Purchase Offer in which to accept the Tag-Along Purchase Offer. In the event that a transfer subject to Section 4.02(a) is to be made to a Person other than a Holder, the original selling Holder shall notify the Proposed Purchaser that the transfer is subject to Section 4.02(a) and shall ensure that no transfer is consummated without the Proposed Purchaser first complying with this Section 4.02(b). It shall be the responsibility of each Holder to determine whether any transaction to which it is a party is subject to Section 4.02(a).

        4.03.     Bring-Along Right.     From and after the date forty-five (45) days after the date hereof, if any Holder of Fund Shares proposes to make a bona fide sale (a " Bring-Along Sale ") of any Restricted Securities held by such Holder to a third party that is not, and following such sale will not be, an Affiliate of the Fund, and such sale is at a price that, in such Holder's reasonable judgment, is a fair market value price, such Holder shall have the right, exercisable upon fifteen (15) days' prior written notice to the each of other Holders, to require each of such other Holders to sell such Holder's Proportionate Share of Restric